review: aggregate comment-size budget guard

[JSONbored]

Closes #6069. Last item in #6065.

Stacked on #6086 (the per-blocker fix-context PR) -- same file (unified-comment-bridge.ts), and this guard needs to wrap the render call site that PR touches.

Summary

Up to 18 optional collapsibles can appear on one comment (Nits, Copy-for-AI-agents, Flagged checks, Linked issue satisfaction, Review context, Contributor next steps, Signal definitions, Chat, Manifest validation, Auto-merge readiness, Changed files, Finding categories, Impact map, Fix handoff, Visual findings/preview, Scroll preview), each capping only itself (Changed files alone up to 200 rows / 30KB). Nothing checked the aggregate size before handing the body to GitHub — a maximally-featured PR could silently exceed GitHub's real ~65,536-character comment cap and fail to post, with no proactive truncation anywhere in the path.

• Added renderWithinBudget: renders the comment; if it exceeds a 60,000-char budget (safe headroom under GitHub's real cap, matching the same limit MAX_STORED_BODY_CHARS in src/db/repositories.ts already codifies for incoming bodies), it drops the lowest-priority optional collapsibles from the end of the chain, one at a time, re-rendering and re-measuring each time, until it fits or none are left to drop.
• "Lowest-priority" = last in buildUnifiedCommentBody's own construction order, which already puts the heaviest/most decorative sections last (Visual preview / Scroll preview's image tables, Changed files' up-to-200-row table) — no new priority list needed, the existing chain order already sorts roughly biggest/most-optional-last.
• Disposition-relevant content — headline, chips, verdict, blockers, per-blocker AI fix context, decision drivers — is rendered directly by renderUnifiedReviewComment, never part of extraCollapsibles, so this loop cannot drop it, by construction (not just by convention).
• When trimming happens, a small "Some detail omitted" note collapsible is appended so the omission is visible in the comment itself to whoever reads it, rather than only in an ops log nobody watching the PR would see.

Test plan

• npx vitest run test/unit/unified-comment-bridge.test.ts (+ the other comment-rendering suites) — 87/87 (+253 total across the related files), including 3 new tests: a normal-sized comment is completely untouched, a 75KB-of-optional-content comment gets trimmed with the note appended and core content intact, and a single collapsible bigger than the whole budget still returns a valid body
• npx tsc --noEmit clean
• npm run docs:drift-check, npm run manifest:drift-check, npm run command-reference:check — pass
• Coverage-checked the exact diff hunks — no uncovered statements/branches within changed lines

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.24%. Comparing base (c1f0695) to head (cd05d7f).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6095   +/-   ##
=======================================
  Coverage   95.24%   95.24%           
=======================================
  Files         595      595           
  Lines       47062    47073   +11     
  Branches    15021    15025    +4     
=======================================
+ Hits        44824    44835   +11     
  Misses       1493     1493           
  Partials      745      745           

Flag	Coverage Δ
shard-1	43.97% <50.00%> (+<0.01%)	⬆️
shard-2	36.61% <45.00%> (-0.01%)	⬇️
shard-3	31.98% <45.00%> (+<0.01%)	⬆️
shard-4	33.84% <100.00%> (+0.01%)	⬆️
shard-5	31.46% <45.00%> (+<0.01%)	⬆️
shard-6	44.84% <35.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/review/unified-comment-bridge.ts	99.60% <100.00%> (+0.01%)	⬆️