fix: disable tunnel autosync by default

[timeleft--]

Summary

• disable tunnel-managed autosync by default with sync interval 0
• make disabled sync mode bypass startup/background sync and omit stale/dirty autosync health fields
• compute tunnel revision freshness from JJ main vs main@origin instead of colocated Git HEAD

Verification

• uv run pytest tests/test_tunnel_cli.py -q
• uv run pytest -q
• git diff --check