feat(channel): gate sensor startup on end-to-end channel handshake

[daibor]

Summary

• Boot sends a single channel notification 5s post-init asking Claude to call the new confirm_channel_received tool. The ack is the proof that notifications/claude/channel is being delivered, so sensor polling (and the dedup state in ~/.world2agent/state.json) only starts on a known-good channel — every channel-enabled session opens with a clean baseline and no signals get silently swallowed by stale seen_ids from a previous non-channel launch.
• Adds a "ready" channel notification right after applyConfig succeeds with the running sensor list, so users see one short Claude-authored confirmation that World2Agent is up and which sources it's watching.
• start_sensors / reload_sensors await the same gate and return a clear help message if the channel never acked, so /world2agent:sensor-add can't half-write state in a non-channel session.
• Persistent boot/lifecycle log at ~/.world2agent/channel.log for inspecting the handshake trace without re-instrumenting the bundle.
• Bumps plugin manifest to 0.1.0-alpha.2.

Why the 5s delay

Empirically, channel notifications fired in the first ~1ms after oninitialized go missing from the session transcript — Claude Code finishes wiring up its channel listener slightly after the MCP init handshake completes. Notifications fired ≥100ms post-init are reliably consumed; 5s gives the listener plenty of headroom, and the confirm_channel_received ack typically arrives ~9s later.

Test plan

• Launch with claude --dangerously-load-development-channels plugin:world2agent@world2agent-plugins, no sensor config changes — verify channel.log shows handshake fire → ack → sensor started, and Claude posts a one-sentence "World2Agent is ready" message.
• Launch plain claude (no flag) — verify the plugin sits idle, sensor does not start, state.json is not written.
• In a plain claude session, run /world2agent:sensor-add — verify the slash command's final reload_sensors call returns the channel-disabled help text and Claude tells the user to relaunch with the flag.
• Tail ~/.world2agent/channel.log and confirm the boot trace lines describe the lifecycle in order (Connecting → Connected → handshake fire → ack → Starting sensor(s) → Started → Sending ready notification).
• After a successful handshake, exit the session — verify the channel subprocess goes away (ps | grep world2agent).

🤖 Generated with Claude Code