fix(mcp): prefilter scoped search_code paths

[yutianyu111602-glitch]

Summary

• Apply path_filter while writing the indexed file list used by scoped search_code grep.
• Keep the existing post-grep path filter as a defensive fallback.
• Normalize indexed Windows paths before regex matching so filter semantics stay aligned with result paths.

Verification

• git diff --check
• wsl.exe bash -lc 'cd /mnt/c/code/githubstar/codebase-memory-mcp && make -f Makefile.cbm cbm -j2'

[yutianyu111602-glitch]

Superseded by #756, which carries the same patch with a DCO Signed-off-by trailer.