fix(search_code): surface cs failures + harden query/path handling

Address findings from an independent code review of the search_code tool:

- Critical: cs failures (non-zero exit, or SIGTERM from the spawn timeout)
  were swallowed and reported to the model as 'No matches found', discarding
  stderr. Now capture exit code + signal from 'close' and return a real
  Error: (timeout message for SIGTERM, exit-code + stderr otherwise). cs
  exits 0 on a genuine no-match, so that path still reports correctly.
- High: a query beginning with '-' (e.g. '-foo') was parsed by cs as a
  (usually invalid) flag. Insert a '--' separator before the query so it is
  always treated as the positional search term.
- Low: relative-path display fallback now matches the workdir only at a path
  boundary, so a sibling dir sharing the prefix (e.g. /app vs /app-secrets)
  isn't rendered as a '../app-secrets/...' path.

Adds tests for the non-zero-exit (stderr surfaced, not 'No matches') and
dash-leading-query cases. All tests (598), biome, and tsc pass.

0 files changed, 0 insertions, 0 deletions