refactor: route supervisor workflow events through chat

[shuxueshuxue]

Summary

Routes supervisor workflow-originated events through the same Mycel backend chat delivery path as normal durable messages.

This removes the remaining content-bearing local terminal hint path from notify_supervisor. Terminal surface delivery remains only for explicit wake controls and backend notification hints.

Architecture Notes

• notify_supervisor(group, message) now sends an addressed chat message to the supervisor user.
• The sender is the logged-in owner account, not the supervisor itself. Current backend semantics skip sender notification and unread for self-sent messages, so supervisor self-send would not wake or read correctly.
• enforce_caught_up=False is used only for this workflow-originated event path because it is a runtime workflow event, not a human/agent conversational send that should be blocked by read-before-send.
• cel group ... supervisor wake still uses direct surface wake because that command is explicitly a wake control, not a durable chat message.

Verification

• uv run ruff check .
• uv run ruff format --check .
• uv run pytest tests/test_group_chat_transport.py -q
• uv run pytest -q -> 468 passed

Follow-up Boundary

This PR does not change backend message typing. The current public send API still records this as the backend's existing send-message type. If workflow message typing needs to become first-class, that should be a backend API primitive rather than CLI-side special casing.

[shuxueshuxue]

Additional YATU Evidence

Ran a real product-path YATU on the macmini in an isolated MYCEL_HOME using a fresh clone of this PR branch.

Artifact: ~/share/mycel-yatu/pr501-supervisor-backend-20260506033947/SUMMARY.md on macmini.

Flow covered:

1. Started isolated Mycel daemon.
2. Created real external supervisor/reviewer identities against https://mycel.nextmind.space.
3. Created a real Mycel group chat.
4. Spawned real Codex supervisor and reviewer sessions in tmux.
5. Ran cel group start <group_id>.
6. Daemon consumed the group-start signal and stored the supervisor event through backend chat.
7. Supervisor inbox received backend chat notifications.
8. Supervisor hook woke the Codex session; the supervisor ran cel read-message and continued workflow.

Key evidence:

• home/daemon.log contains workflow supervisor chat stored [0d09bd98-48dd-4d2e-9a58-c8f5753a1bf4].
• supervisor-read-last.out contains message seq 2 with <cel event="group_start" ...> from owner user.
• The supervisor tmux capture shows hook metadata only, followed by the supervisor using cel read-message for real content.

The first harness assertion had a wrong log path ($MYCEL_HOME/state/daemon.log instead of $MYCEL_HOME/daemon.log), but the product path itself passed. The isolated daemon was stopped afterwards.