Add incremental Matrix sync command

[mdheller]

Summary

Adds incremental Matrix sync support using the durable Matrix sync state introduced in the prior lane.

This PR adds:

• MatrixSyncRequest for incremental sync requests.
• MatrixServiceBackend.sync() protocol method.
• Offline in-memory sync behavior for CI.
• Optional matrix-nio backed sync implementation.
• MatrixServiceAdapter support for backend-driven matrix_sync when no explicit payload is provided.
• agent-term-matrix sync command using stored next_batch.
• Sync options for --timeout-ms, --full-state, --persist, --save-state, and --no-save-state.
• Tests for backend sync, adapter sync, CLI sync, state updates, and persistence behavior.

Boundary decisions

• CI uses the in-memory backend and does not need a homeserver.
• matrix-nio remains optional and lazy-imported.
• Sync state is local durable state, not authority over Matrix room semantics.
• Event persistence remains explicit via --persist.

Validation

Expected CI:

ruff check .
pytest