CLI: add --source filter to `query` and `search` commands

[jeremyknows]

Problem

gbrain query and gbrain search return cross-source results with no way to filter by source_id. When a gbrain brain has multiple ingested sources (wiki articles, user diaries, external docs), queries intended for one source surface results from others.

Current workaround

We filter results client-side by maintaining a slug allowlist of wiki article filenames and discarding any gbrain result whose slug is not in the allowlist. This works but requires N+1 filesystem reads and doesn't scale to sources with non-slug-based identifiers.

Requested

A --source <source_id> flag on gbrain query:

gbrain query "how does Atlas route dispatch events" --source wiki --limit 5

And equivalently on gbrain search. The source_id should match the source identifier used in gbrain sync --source <id>.

Context

• gbrain version: 0.30.2
• Brain has 11 sources, ~3,350 pages; wiki is ~50 pages; watson-diary is ~2,554 pages (76% of total). Cross-source bleed kills wiki recall on infrastructure queries.
• Related: PR feat(sync): --src-subpath + --exclude for monorepo subdir-source support #774 (--src-subpath for monorepo subdir sources)