feat(agents): deduplicate CLAUDE.md and AGENTS.md in agent context

[aaight]

Summary

• Deduplicates CLAUDE.md and AGENTS.md in the agent context — when both files exist and contain identical content (same bytes, or one is a symlink of the other), only injects the content once into the agent prompt
• Detection strategy — symlink fast-path via lstatSync/readlinkSync, falling back to trimmed-content string comparison for copy-paste duplicates or when filesystem calls fail
• CLAUDE.md kept as canonical — when deduplicating, the CLAUDE.md entry is retained (it's the more widely referenced name across prompts/docs)
• Single production file changed — only src/agents/utils/setup.ts (readContextFiles + new areDuplicateContextFiles helper)

Card: https://trello.com/c/H7YTaJag/628-right-now-we-put-agentsmd-and-claudemd-into-the-context-sometimes-they-have-exact-same-contents-checksum-wise-sometimes-one-is-a

Test plan

• New unit tests in tests/unit/agents/utils/setup.test.ts covering:
  • Identical content deduplication keeps CLAUDE.md (content match)
  • Different content → both files retained
  • AGENTS.md is a symlink to CLAUDE.md → deduplicated
  • CLAUDE.md is a symlink to AGENTS.md → deduplicated
  • lstat throws → falls back to content comparison correctly
  • Only 1 file exists → no dedup attempt (lstatSync never called)
• All 31 tests in setup.test.ts pass (25 original + 6 new)
• All 715 agent unit tests pass
• Lint passes (biome check: 0 issues)
• Type check passes (tsc --noEmit: 0 errors)

🤖 Generated with Claude Code

🕵️ claude-code · claude-sonnet-4-6 · run details

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/agents/utils/setup.ts	90.90%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[nhopeatall]

LGTM — clean, focused change. The dedup logic is sound: symlink fast-path via lstatSync/readlinkSync correctly avoids following symlinks, content equality fallback handles copy-paste duplicates and permission errors gracefully. The results.length === 2 guard ensures no unnecessary filesystem calls when only one file exists. Existing tests remain correct because unmocked lstatSync throws → caught → falls to content comparison → different content → both files kept. Six new tests provide solid coverage of the key scenarios (content match, content mismatch, symlink in both directions, lstat failure fallback, single-file short-circuit).

🕵️ claude-code · claude-opus-4-6 · run details