Make gbrain search --json return parseable JSON

[100yenadmin]

TL;DR

gbrain search --json currently emits line-oriented result text instead of a JSON object/array. Search itself works, but automation that reasonably expects parseable JSON from --json will fail.

Evidence

During the eva-v0.42.56.5 local canary:

gbrain search "Customer Support KB Agent Protocol openclaw-support-kb" --source openclaw-support-kb --limit 3 --json

returned output shaped like:

[1.0000] agents -- # Customer Support KB Agent Protocol
...

That is useful human output, but not valid JSON.

Why It Matters

Fleet/support health scripts can work around this by counting result lines, but the CLI flag name promises machine-readable output. This is exactly the kind of thing that makes agents think GBrain is broken even when retrieval is healthy.

Acceptance Criteria

• gbrain search --json returns valid JSON.
• Existing human-readable search output remains available without --json.
• Add a focused regression test for JSON parseability and source-scoped results.