perf(mcp): prefilter scoped search_code filelist by path_filter

[DeusData]

perf(mcp): prefilter scoped search_code filelist by path_filter

Distilled from #756 (author: yutianyu111602-glitch), rebased onto the post-#751
filelist writer (NUL-delimited on Unix / newline on Windows, single fwrite loop).

What

Scoped search_code now applies path_filter while WRITING the indexed
filelist (write_scoped_filelist), instead of only after grep has scanned
every indexed file. On large indexed projects with a narrow path_filter,
grep no longer scans files whose hits the post-grep filter would discard
anyway.

Results-preserving by construction

The prefilter predicate is IDENTICAL to the post-grep filter in
collect_grep_matches: the same compiled path_regex (cbm_regcomp with
CBM_REG_EXTENDED|CBM_REG_NOSUB) run via the same cbm_regexec call against
the same root-relative path string (Windows separators normalized with
cbm_normalize_path_sep first, mirroring the post-filter). It can only
remove filelist entries; nothing new reaches the shell. The post-grep filter
is kept as belt-and-suspenders (per #756).

Empty-filelist guard

The prefilter introduces a previously unreachable edge: a path_filter
matching zero indexed files yields a 0-record filelist. xargs behavior on
empty input is platform-dependent (GNU execs grep once with no operands, BSD
skips). handle_search_code now short-circuits: when the scoped filelist has
0 records, the grep subprocess is skipped and the normal empty result is
returned directly. Verified empirically: on main this edge cannot occur (the
full filelist is always written and the post-filter drops all hits), so this
is a guard for the new edge, not a bug fix on main.

Tests (the original PR had none)

• search_code_path_filter_prefilter_keeps_matches — positive invariant
  guard: a path_filter matching the file containing the hit still returns
  that hit (total_grep_matches == 1), the out-of-filter file is excluded.
  Green on main AND with the fix — predicate-identity / no-over-filtering
  invariant for a perf-only change.
• search_code_path_filter_matches_nothing — path_filter matching zero
  indexed files returns a clean zero-result response (no error,
  total_grep_matches == 0, total_results == 0). Green on main too
  (grep runs and the post-filter drops everything there); with the fix the
  grep subprocess is skipped entirely (observable: 6ms → 0ms in the suite).
• A prefilter write-count log assertion was considered and skipped:
  write_scoped_filelist has no existing count log line, and adding logging
  just for a test was out of scope.

No linked issue — no Closes.