Send real task progress and completion notifications back to Matrix

[chindris-mihai-alexandru]

Summary

/build and /plan currently promise that the bot will notify the user when a task completes, but _monitor_events() only logs assistant events and never posts a real completion update to Matrix.

Why this matters

This is core product trust. If remote execution starts from a phone, the user needs a reliable end-of-task signal without checking logs or polling manually.

Proposed scope

• Subscribe to OpenCode session events and detect state transitions such as running, waiting, completed, failed, and aborted.
• Post concise progress updates back to the originating Matrix room.
• Include the final assistant summary or a short completion digest when available.
• Avoid duplicate notifications during reconnects or event replay.
• Chunk long responses safely using the existing message splitting logic.

Acceptance criteria

• A /build or /plan request produces a Matrix completion or failure message in the same room.
• Notification routing works after reconnects and bot restarts.
• Duplicate event deliveries do not spam the room.
• Failure states are surfaced with enough detail to debug from mobile.