docs(changelog): fix unreleased release notes for the next release

[code-yeongyu]

Summary

The next Senpi release's changelog audit (v2026.8.16..main, per .github/agent/commands/cl.md) found two defects in the unpublished [Unreleased] release notes, both in packages/coding-agent/CHANGELOG.md. This PR fixes exactly those two and nothing else: a broken PR link placeholder that would have shipped verbatim in the next GitHub release body, and a missing cross-package entry required by the changelog guide's duplication rule.

Changes

Both edits are confined to the [Unreleased] section; no released section is touched.

• Link the cursor-cli-oauth entry to its PR. The entry ended with the literal placeholder ([#PR-NUMBER](https://github.com/code-yeongyu/senpi/pull/PR-NUMBER)). PR feat(coding-agent): cursor-cli-oauth — optional Cursor CLI fallback lane #921 ("feat(coding-agent): cursor-cli-oauth — optional Cursor CLI fallback lane", merged 2026-08-17) is the sole owner of that entry, so the placeholder is replaced with [#921](https://github.com/code-yeongyu/senpi/pull/921).
• Duplicate the codex -fast catalog addition into the user-facing package. PR feat(ai): generate -fast variants for openai-codex priority-eligible models #918 added openai-codex -fast priority-tier variants (generator + regenerated catalog data) and recorded it only in packages/ai/CHANGELOG.md. The changelog guide requires ai changes that affect end users to be duplicated to the coding-agent changelog — the new variants are user-selectable models. Precedent: the original openai -fast variant addition was recorded in this changelog (released section, [#420](https://github.com/code-yeongyu/senpi/pull/420)). One concise ### Added bullet linked to feat(ai): generate -fast variants for openai-codex priority-eligible models #918 is appended to the existing populated ### Added block.

Audit summary (full table in the task evidence)

All other merged PRs in the window are already covered: #909 (tui + coding-agent), #910 (ai + agent + coding-agent), #911 (coding-agent), #915 (ai + coding-agent), #916 (senpi-codemode), #917 (coding-agent), #922 (coding-agent). #919 is scripts/docs only and needs no package entry.

Two pre-existing defects are deliberately not edited here and flagged for the release owner:

1. PR fix(coding-agent): keep custom-editor Enter submissions from being dropped #908's entry sits under the released ## [2026.8.16] heading but is not in the v2026.8.16 tag (post-tag merge placed it after the section rename); it also lacks the by [@Tinycute00] attribution. Moving or editing it would mutate a released section, which the changelog guide forbids.
2. The coding-agent [Unreleased] skeleton carries trailing empty headings (### New Features, ### Breaking Changes, ### Added, ### Changed, ### Removed) introduced by release-bot commit 4603673f0; reAddUnreleasedSections re-emits the previous cycle's captured (already malformed) subsection list. Cosmetic in rendered notes; belongs with release tooling, not this prose PR.

QA & Evidence

• What was tested: the exact release-notes generator CI uses (node scripts/release-notes.mjs extract --version Unreleased --repo code-yeongyu/senpi), the same command build-binaries.yml runs to produce RELEASE_NOTES.md.
  • RED (before): rendered body contains [#PR-NUMBER](https://github.com/code-yeongyu/senpi/pull/PR-NUMBER); target returns HTTP 404.
  • GREEN (after): zero PR-NUMBER occurrences; pull/921 and pull/918 each appear once; both targets return HTTP 200.
• What was tested: git diff --check — clean (no whitespace errors).
• What was tested: the PR changelog gate locally with PR-equivalent env (CHANGELOG_GATE_BASE=<main sha> node scripts/check-pr-changelog.mjs) — PASS - no runtime source changes detected.
• Self-review: single file, one hunk inside [Unreleased]; released ## [2026.8.16] section untouched; no new headings, no duplicate headings.
• Artifacts: local-ignore/qa-evidence/20260817-senpi-omo-beta-release/01-senpi-changelog/ (worktree context, full audit table, RED preview with link probes, GREEN preview + verification, final diff).

Risks & Residuals

• Prose-only, no runtime surface: documentation-only change per AGENTS.md requires focused document validation (release-notes preview + gate + git diff --check), no runtime QA. Mitigated by the evidence above.
• The two flagged pre-existing artifacts (fix(coding-agent): keep custom-editor Enter submissions from being dropped #908 placement, malformed skeleton) remain; editing them is either forbidden (released-section immutability) or release-tooling scope. Accepted as out of scope for this PR.

Related Issues

Release-note gate for the next CalVer release; unblocks the release audit for v2026.8.16..main.

---

Summary by cubic

Fixes two defects in the coding-agent [Unreleased] changelog so the next release body renders a valid link and includes a required user-facing entry.

• Links the cursor-cli-oauth entry to PR feat(coding-agent): cursor-cli-oauth — optional Cursor CLI fallback lane #921 instead of shipping a [#PR-NUMBER] placeholder.
• Adds the openai-codex -fast model variants entry (PR feat(ai): generate -fast variants for openai-codex priority-eligible models #918) per the cross-package duplication rule.

Changes are confined to [Unreleased] in packages/coding-agent/CHANGELOG.md; no released sections or runtime behavior are affected.

^{Written for commit 7bdef48. Summary will update on new commits.}

[code-yeongyu]

CI note for reviewers: the first Check and test run failed Test (coding-agent 2/3) at test/cursor-cli-oauth/shutdown.test.ts:85 — "Error: process 0 survived the shutdown grace window". This diff is 4 prose lines in one CHANGELOG.md and has no mechanism to affect process lifecycle; the same job passed on the exact base commit e67359ec0 (run 32051597745) and on every other completed main run checked (32043904967, 32041228235, 32039722931, 32038921083, 32033858665). Treated as a timing flake; rerun of the failed jobs completed success (run 32052771724, head_sha 7bdef48). Current state: Changelog gate pass, Check and test pass, all other checks pass.