carryForwardComments() only processed markdown files, leaving code files
to fall through to carryForwardAllComments() which copies at original
positions with zero remapping. This meant code file comments pointed to
wrong lines after agent edits, and comments on deleted lines became
invisible.

- Unified carryForwardFileComments handles all file types with LCS line
  mapping + anchor verification (previously markdown-only)
- Snapshot PreviousContent for all files with comments, not just markdown
- Fix old-side anchor extraction: read from base ref via git show instead
  of working tree (which doesn't contain deleted lines)
- Add fileContentAtRef() helper for reading file content at a git ref

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…rward

Old-side comments reference the base ref (stable across rounds), not the
working tree. Running them through LCS remapping produced meaningless
results and incorrectly marked them as drifted.

- Skip LCS remapping and anchor correction for old-side comments
- Add test for old-side comment position preservation
- Fix stale comment about "anchor-only search"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>