Fix FileIndexer race and symlink handling

[Widthdom]

Summary

• Add mtime recheck/retry when indexing file content.
• Treat mid-scan delete races as non-fatal skipped paths and keep purge authority.
• Report dangling directory symlinks and expose dangling_symlinks_skipped in full-scan JSON.
• Add --follow-symlinks none|internal|all with none as the default.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~FileIndexerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted, but local git signing configuration caused git-commit-based tests to fail with an SSH signing key passphrase prompt.

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog fragments:
  • changelog.d/unreleased/1654.fixed.md
  • changelog.d/unreleased/1655.fixed.md
  • changelog.d/unreleased/1656.fixed.md
  • changelog.d/unreleased/1711.added.md

Notes

• Issue Ignore-file TOCTOU between exists check and read leaves stale rules in scan window #1653 was excluded because it already had a work-started comment.
• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.
• Commit granularity does not match the requested one-issue-per-commit split; this branch currently has one combined implementation commit.

Fixes #1654
Fixes #1655
Fixes #1656
Fixes #1711