feat: add channel event notification system

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Summary

• Add Channel module (src/channel/) that pushes platform event notifications into active Claude Code sessions via MCP Channel protocol
• Support two event source types: polling (periodic opencli command execution with diff detection) and webhook (HTTP POST receiver)
• Register opencli channel start/status/stop subcommands

Features

• Polling source: Wraps any opencli command as a periodic event source, detects new items via snapshot diffing
• Webhook source: HTTP server on localhost accepting POST events with optional Bearer token auth
• Event queue: Bounded (200 events) with dedup window (500 IDs) to prevent duplicate notifications
• Single-instance lock: PID-based lock file with stale detection
• Browser lock: Cross-process coordination between channel polling and interactive opencli usage
• MCP Channel protocol: Declares claude/channel experimental capability, pushes via notifications/claude/channel

Usage

# 1. Configure ~/.opencli/channel.yaml
# 2. Register MCP server in ~/.claude.json
# 3. Launch: claude --dangerously-load-development-channels server:opencli-channel

See docs/advanced/channel.md for full documentation.

Test plan

• Unit tests: EventQueue (6/6), PollingSource (7/7), WebhookSource (5/5)
• E2E test: channel status lifecycle
• TypeScript typecheck: zero errors
• Manual test: webhook notification received in live Claude Code session
• Manual test: v2ex/hot polling notifications received in live Claude Code session