Add Antigravity peer and lane orchestration support

[antst]

Summary

• add the native agy-peer launcher, process-attested launch records, Agy hooks, and agent_sessions MCP surface
• publish Agy conversations as Claude-compatible local peers with bidirectional hook-boundary delivery
• let attested Agy conversations own and operate the existing Codex and Claude lane CLIs without adding another lifecycle engine
• factor launch-token identity and product registration so the upcoming Grok adapter can reuse shared policy
• select Agy fail-closed: validate the headless CLI contract, reject GUI helpers and symlink chains on Linux and macOS, and preserve native argv
• install the Agy plugin directly and transactionally, register its import manifest without invoking GUI-delegating agy plugin commands, and preserve unrelated manifest data
• derive Agy MCP caller identity from the attested host so session_id is optional but can never select another conversation

Validation

• uncached scripts/test and RACE=1 scripts/test, including federation integration
• Go vet; golangci-lint 2.12.2: 0 issues
• Linux amd64/arm64 and Darwin amd64/arm64 cross-builds
• real headless Agy 1.1.13 resolver plus synthetic Linux and macOS GUI-helper rejection without executing the helper
• live Agy identity/list/send round trip and post-hook delivery
• live interactive Agy MCP on successive later turns: two process-attested rename_session calls without session_id, with both durable state changes verified and no shell fallback
• live Agy-owned Codex lane run/collect/archive
• Agy-owned Claude lane lifecycle ran and archived correctly; the worker response failed because this host Claude OAuth session is expired

The user-owned untracked .grok/ directory and docs/GROK-ADAPTER.md are deliberately excluded from this PR.

[antst]

Parking this PR as an open draft. The stock Agy CLI adapter provides attested discovery, durable delivery, outgoing messaging, and Codex/Claude lane ownership, but it cannot wake a fully idle interactive Agy prompt without human input. That fails the core reactive-peer requirement.

The continuation memo is now in docs/ANTIGRAVITY-ADAPTER.md (commit 22dfe10). It records verified behavior, rejected schedule/MCP/PTY/concurrent-writer approaches, the official SDK trigger investigation and its separate-auth/billing constraint, and the recommended subscription-compatible agy-peer-lane architecture using serialized headless CLI turns.

The product-independent Codex resume-CWD fix will be extracted into a separate PR. This PR remains open to preserve the implementation and review history for future continuation.