chore(core): retire dead context/inject.ts twin (T-605/T-606, ADR-049)

[matze4u]

What

Retire the dead context-injection twin packages/core/src/context/inject.ts (+ its test), superseded by the live buildContextInjection.

context/inject.ts (buildContextBlock / injectContext, user-message placement) was the recorded implementation of T-605 (context-block injection) + T-606 (cache-friendly placement), but had zero live callers — referenced only by its own inject.test.ts. The live path is chat/context-injection.ts::buildContextInjection (T-MR13, ADR-025), which folds the snippets into a bounded <workspace-context> system-prompt block, used in both chat/handler.ts:356 and agent/turn-handler.ts:312. The system-prompt placement won; the user-message twin is the dead loser.

Same retire shape as PR #51 (dead tracking/audit-log.ts, T-413, ADR-039) and exactly the cleanup ADR-048 named when it flagged context/inject.ts as the one genuinely dead twin.

Why it is safe

• Grep across packages/core/src + clients/ (excluding its own test and /dist/): zero callers. No barrel/index re-exports.
• Deleting it does not orphan context/snippet.ts — Snippet is still imported by annotations.ts, indexer.ts, chunk-tree.ts, engine.ts.
• No protocol/codegen change; no runtime behavior change.

Both impls preserve the same cache goal

Keep the cached static prefix byte-identical, let the per-turn context ride where cache misses are expected — inject.ts via the user message, buildContextInjection via the system-prompt suffix after the cache_control'd guidelines/rules prefix. No insight lost; the live placement additionally gives better instruction-following steering.

AI council

codex-cli 0.134.0 + gemini-cli 0.41.2 (2026-06-02) — UNANIMOUS:

• Q0=A retire the zero-caller competing contract
• Q1=A dedicated ADR-049 (mirrors the ADR-039 retire precedent)
• Q2=A T-605/T-606 stay [x], done-comments updated to cite the live impl
• Q3=A nothing to port (the fixed 8000-char cap supersedes the dynamic 20%-window budget; safer against system-prompt cache-block overflow)
• Q4 no trap

Changes

• chore(core): delete context/inject.ts + context/inject.test.ts.
• docs: ADR-049; index.md row; road-to-v1-0.md T-605/T-606 done-comments updated (stay [x] — the capability is live via the system-prompt placement).

Verification

task ci deterministic phases (lint, build, typecheck, format) green every run. Core test suite 3/4 full runs green (1120 passed / 1 skip; -6 = the removed inject.test.ts).

The 1/4 failure is a pre-existing, intermittent flake unrelated to this diff: chat/handler-list.test.ts (conversationList newest-first order, PR #58 / ADR-046) trips on a same-millisecond timestamp tie under full-suite load — it passes 3/3 in isolation, and sessions/watcher.test.ts (chokidar debounce) rotated into the same slot on one run (passes 2/2 in isolation). This diff touches none of those modules. Remote CI (6-node matrix) is the authoritative gate.

No checkbox flip — T-605/T-606 stay [x].