cleanup: extractKeywords() in recall-gate.ts is dead code (orphaned by the ILIKE removal)

`extractKeywords()` (`src/hooks/shared/recall-gate.ts:153`) is documented as:

> ```
> // Extract salient lower-cased keywords from a prompt for the lexical fallback.
> ```

The lexical fallback was removed in `bae7bbb1` ("semantic-only proactive recall, drop ILIKE fallback"). `e98aa533` cleaned up the stale comments in `recall.ts` but left this helper behind.

It is still exported and still tested (`tests/shared/recall.test.ts:495-511`), but has **zero production callers** — the only references outside its own definition are the test file.

Low priority. Two reasonable resolutions:
- Delete the function, its `STOPWORDS` set if unused elsewhere, and the describe block; or
- Keep it and fix the comment to note it's retained for a possible future lexical tier, so the next reader doesn't assume there's a live lexical path in proactive recall.

Either is fine — the current state is just misleading, since the docstring describes a code path that no longer exists.