Detect malformed WarpGrep Windows results and harden error formatting

Fixes #7 — On Windows, upstream SDK path parsing truncates drive-letter
paths (e.g. C:/Users/.../auth.ts → file: 'C'), producing confusing
output. This adds detection for malformed contexts and surfaces an
actionable error with a workaround suggestion. Also fixes the
'Search failed: undefined' output when the SDK omits an error string.

https://claude.ai/code/session_01G5qa1GyHGNwWu2ko6uZ53N