feat(matrix): add session-local replies and terminal silence

[arcuru-bot]

This replaces #77 with a session-local Matrix room handle. It stands alone on main; #77 does not need to land first.

• A session attaches to at most one Matrix login and room. Eligible messages retain their source; addressed messages wake the agent, and ambient group participation requires an authorized opt-in.
• A normal final from a Matrix-origin turn is saved locally and queued to that room. The final, addressed outbox row, and completed turn commit together. Local, scheduled, and agent-to-agent turns do not auto-post to Matrix.
• no_reply({}) ends a turn without a final or room post. It is available on every turn in a Matrix-bound session so the model's tool list stays stable. The explicit matrix__send tool remains available for proactive posts; using it during a Matrix turn suppresses automatic posting of the later final.
• The bridge verifies the binding at delivery, persists send progress across restarts, and records a receipt. Other transports do not re-send the local copy of a Matrix reply. A room must opt in again under the new participation setting; the earlier pilot setting is not reused.

The disposable Matrix E2E covered normal finals, terminal silence, explicit sends without duplicate posts, restarts, rejoin and delivery counts. In a manual room test, two replies appeared once each and a no_reply turn made no post.