feat(session): add immutable task owner protocol codecs

[snowykr]

Summary

• Add complete strict immutable task-owner locator, deletion evidence, native disposition and shrinking-continuation DTO codecs.
• Pure protocol code only: no filesystem/native/store effects, SessionManager/SDK/GC wiring, owner creation or task producer activation. Decoded data never grants deletion permission or physical-completion/writer-quiescence proof.
• Reject unknown keys, malformed canonical identities/hashes, mismatched session/parent/root, expanded authority and contradictory native flags.

Review range and sequencing

• Base: dev, captured at 8e709f5964987ab7dc206ca9d4b86c9e039f3803.
• Exact head: c2f29e0894f103a4872628eda481fd8482ff2933.
• Actual dev diff AND owned production diff: 578 additions + deletions; tests separately add 314 lines. Two files total.
• This codec was first independently qualified directly on dev without the unrelated retention extraction. refactor(session): separate retention orchestration from storage #6282 then externally merged the exact approved 85155636fe40056498a99674bdb90aa338ef1568, not the later prepared branch head. This PR now contains that actual merge through dev ancestry and was freshly rebased and qualified on 8e709f5964987ab7dc206ca9d4b86c9e039f3803. Its production review range remains 578; the earlier oversized cumulative prepared branch is not presented as a small PR.
• Later access, provisioning, native retirement and journal consumers depend on this codec and require their own exact-head qualification. All automatic producers remain disabled until complete consumers are present.

Verification (exact clean tracked source, independent QA)

• bun test packages/coding-agent/test/task-artifact-owner-codec.test.ts packages/coding-agent/test/session-storage.test.ts packages/coding-agent/test/gc-disk-retention.test.ts — passed.
• bun --cwd=packages/coding-agent run check — vendor, Biome and TypeScript passed; pre-existing warnings/info preserved.
• Real Darwin ARM64 native addon matched committed strict 0.18.5 metadata and native source; metadata was not rewritten. Source clean before/after qualification.
• Execution report: 04-owner-codec-merged-retirement-qualification.json; full output retained alongside it. Internal reports are evidence retention, not public CI substitutes.

Limits

Darwin-focused API/package proof, not a full task-move/persistent-owner or public SDK deletion end-to-end claim. Existing platform skips/warnings remain uncredited. No physical reclamation or writer revocation claim. Exact-head code-event Dev CI 37139616416 completed successfully at c2f29e0894f103a4872628eda481fd8482ff2933. Metadata-only 37139647696 was skipped and is not code evidence; old-head b550… run 37139119934 was cancelled and is not green. Exact-head write-access maintainer review remains required (REVIEW_REQUIRED at capture); old reviews are not transferred. No leader merge.

No changelog fragment: this inert internal DTO module does not change observable runtime behavior. Original 44-file staged candidate and consolidated #6240 reference remain untouched and are not incorporated wholesale.

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[probepark]

Review (head c2f29e0, gajae-reviewer on behalf of probepark) — Approve held

Approve held: CI plan did not run cd packages/coding-agent && bun run check on c2f29e0; run it at this head (or add it to the plan) and re-request review.

The head's affected-path plan has no check:@gajae-code/coding-agent shard, so biome + check:types never ran in CI for this head. That matters here specifically: the new module is ~578 lines of type-narrowing over unknown (Record<string, unknown> property narrowing, as const tuple iteration at task-artifact-owner-codec.ts:510-518, the as NativeExactUnlinkResult cast at :550) — exactly the surface tsc --noEmit vets. The PR body reports a local bun --cwd=packages/coding-agent run check pass, but it does not tie that run to c2f29e0 with the command's exit code. A reply showing that command run at this exact head with exit 0 is enough to release the hold.

CI: 0 pending, 0 failed (gate checks excluded); typecheck shard absent from plan — see above.
Scope: +892 / -0, 2 files — packages/coding-agent/src/session/task-artifact-owner-codec.ts (+578, new, no production importer yet), packages/coding-agent/test/task-artifact-owner-codec.test.ts (+314).
Conventions: no changelog fragment (justified in body: inert internal DTO module); no generated files; no console.*; no new Worker; no labels.
Notable:

• task-artifact-owner-codec.ts:240-266 — retainedTreeDoesNotExpandAuthority duplicates the helper of the same name in session-storage.ts but adds a stricter mtime/ctime equality check when sha256 is absent. Two diverging copies of an authority check is a drift risk once consumers land; consider making one delegate to the other.
• Code review found no blocking candidates: unknown keys, non-canonical u64 ids, path traversal, parent/root mismatch, tree expansion and contradictory native ok/code/payloadDurable flags all fail closed.
  Blocking: none from code review; verdict withheld only for the missing typecheck run.

[snowykr]

Exact-head package check proof for the held review:

head: c2f29e0894f103a4872628eda481fd8482ff2933
checkout: clean tracked source, independent /private/tmp/gjc-task-stack-native-qa
command: bun --cwd=packages/coding-agent run check
exit code: 0
included: bun run verify:markit-vendor && biome check . && bun run check:types
TypeScript command: tsc -p tsconfig.json --noEmit
tracked checkout remained clean before/after

This is the already-executed current-head qualification, not an inference from CI or a later consumer head. The recorded 04-owner-codec-merged-retirement-qualification.json binds head to the exact full SHA above, packageCheckExitCode: 0, status: passed, and checkoutExactBeforeAfter: true; the full command output is retained in the matching -qualification-output.txt. Focused codec/storage/GC tests also exited 0 at that same head. Native addon matched committed strict 0.18.5 metadata; no trusted metadata rewrite.

Existing Biome large-file/unused-loader warnings and String.raw info were retained, not suppressed. The CI typecheck-shard omission is acknowledged; code-event CI success alone was not used as package-typecheck proof. Please re-review this exact head using the explicit exit-0 package result.

The duplicate shrinking-tree check is recorded as an advisory integration risk; later consumers must not silently weaken its stricter timestamp/authority behavior. This inert root introduces no production importer or automatic owner producer.

[probepark]

Review (head c2f29e0, gajae-reviewer on behalf of probepark)

need-local satisfied by #6283 (comment) — author ran bun --cwd=packages/coding-agent run check (verify:markit-vendor + biome check . + check:types / tsc -p tsconfig.json --noEmit) at exact head c2f29e0894f103a4872628eda481fd8482ff2933 on a clean checkout, exit code 0. This was the only condition of the earlier hold (review 5401970991).

CI: green — 0 pending, 0 failed (gate checks excluded); the plan omitted the check:@gajae-code/coding-agent shard, which the local exact-head run above covers.
Scope: +892 / -0, 2 files — packages/coding-agent/src/session/task-artifact-owner-codec.ts (+578, new, no production importer yet), packages/coding-agent/test/task-artifact-owner-codec.test.ts (+314). Reviewable source 578 lines (test excluded).
Conventions: no changelog fragment (justified in body: inert internal DTO module); no generated files; no console.*; no new Worker; no labels.
Notable:

• task-artifact-owner-codec.ts:240-266 — retainedTreeDoesNotExpandAuthority duplicates the session-storage.ts helper with a stricter mtime/ctime check when sha256 is absent; the author has recorded this as an advisory integration risk. When consumers land, have one copy delegate to the other.
  Blocking: none.

PR body verdict line count=0, not updated. Suggested verdict line: gajae.pr-review-verdict.v1 merge-approved sha256:ca1145415b4a50f7d775a38a044bd6b8259e6e66e3bdee390d1515dff06ec016 reviewer:human reviewer-id:probepark evidence:ci-green;code-reviewed-no-blocking;need-local-check-exit0-at-head-by-author-comment-5972271030

Verdict: gajae.pr-review-verdict.v1 merge-approved sha256:ca1145415b4a50f7d775a38a044bd6b8259e6e66e3bdee390d1515dff06ec016 reviewer:human reviewer-id:probepark evidence:ci-green;code-reviewed-no-blocking;need-local-check-exit0-at-head-by-author-comment-5972271030

[probepark]

Merged to dev: approved at c2f29e0, checks green, no open concerns.