v1.5.3 — Antigravity connector: stdio MCP server mode

The governed Antigravity/Gemini connector gains a stdio MCP server mode, so an external stdio MCP client can connect to it directly rather than driving the one-shot --list/--call CLI. Contributed through the Antigravity branch and accepted into main after HQ review and verification under the ring governance. Still 82 MCP tools; governed surface remains 41.

Added

• sovereign_connector.py — run_proxy / --server: a thread-based stdio MCP proxy server. Spawns the local sovereign, forwards the MCP handshake, filters tools/list via governed_tool_list, and routes tools/call through governed_call — Ring 1 forwarded to the spawned sovereign, Ring 2 → pending proposal, Ring 3 blocked. Reuses the canonical bridge_core.rings; no separate ring list.
• --source-instance env fallback: $SOVEREIGN_SOURCE_INSTANCE → source-instance env key → antigravity-connector. CLI flag still overrides.
• README updated with the MCP-server registration snippet and the Honesty Contract pending-proposal flow.

Verified (HQ)

Proxy tools/list = 41, zero Ring 3 leak; Ring 3 (record_insight) blocked with the surface message; Ring 2 (propose_insight) → pending proposal, not the chronicle; env-var attribution confirmed. Heartbeat reports 1.5.3 / 82.

External substrate output, reviewed and gated through the ring membrane. The Claude exemption keeps the trust boundary explicit: external substrates are ringed; Claude is not.