test: investigate subscription-backed Antigravity CLI bridge

[rynfar]

Antigravity needs a subscription-account CLI transport before it can serve as a Meridian backend. This draft adds an opt-in research harness and recorded live evidence; production Meridian behavior is unchanged.

The official agy CLI (1.2.6, macOS arm64, Gemini 3.8 Flash Low) completed streaming, explicit resume, persistent stdin, concurrent conversations, and a synthetic HTTP client-owned tool round trip through a pending MCP response. A fresh process recalled the completed tool result. No API-key fallback, SDK, private model endpoint, credential copying, or transcript editing is used.

Remaining limits are explicit: native tool_result stdin is rejected, successful tool transport currently requires per-process auto-approval plus a restrictive hook, and answers include unsolicited timestamps. A built-in file read was denied by that hook even under auto-approval. Actual client integration, scoped grants, pending-call recovery, images, and undo/compaction remain unproven. This is research, not a production-ready provider.

Validation:

• Live probes and sanitized evidence: docs/research/antigravity-cli.md and antigravity-cli-evidence.json.
• Script syntax, typecheck, build, JSON/redaction, local links and diff checks passed.
• npm test: 3,677 passed, 2 skipped, 2 failed in unchanged session-GC process-kill tests (EPERM). Both failures reproduced in a focused run; later chained stages did not run. No production or existing test files differ from base 4c9ebbb6.

Keep this PR in draft for feasibility review. No merge or release is intended.