Add Matrix adapter scaffold

[mdheller]

Summary

Adds a dependency-free Matrix adapter scaffold for AgentTerm.

This PR replaces stale PR #12 with the same tested Matrix normalization and posture boundary rebased on current main after the Policy Fabric scaffold merge.

It adds:

• MatrixE2EEPosture for encrypted-room posture decisions.
• MatrixRoomEvent normalized event model.
• MatrixAdapter for room-event normalization, E2EE posture checks, and prepared emit events.
• Normalization for room ID, alias, event ID, sender MXID, event type, thread root, redactions, membership changes, bridge metadata, and E2EE posture.
• Tests for threaded messages, bridge metadata, redactions, membership, sensitive-context blocking, verified encrypted rooms, and emit preparation.

Boundary decisions

• Matrix remains the canonical ChatOps transport.
• Slack/Discord remain bridge targets represented as metadata, not authority.
• AgentTerm preserves Matrix metadata instead of flattening messages into generic chat.
• Sensitive context release fails closed when encrypted-room posture is unknown or unverified.
• This does not add live homeserver network I/O yet; it creates the contract and test harness for that next step.

Validation

Expected CI:

ruff check .
pytest

Part of #2. Replaces #12.