feat: completion notifications (Slack, Discord, Telegram)

[JeremyDev87]

Purpose

Send notifications when long-running tasks complete — parallel execution waves, AUTO mode cycles, /ship results. Essential for async workflows where user walks away.

Changes

• codingbuddy.config.json: notification config (webhook URLs, channels)
• Notification service in MCP server
• Integration points: /ship completion, Wave completion, EVAL results

Acceptance Criteria

• Slack webhook notification support
• Discord webhook notification support
• Telegram bot notification support
• Configurable per-event notification (ship, wave-complete, error)
• Message includes: task summary, PR URL, cost, duration

[JeremyDev87]

Superseded by #829 (plugin event-driven notifications). Plugin triggers notifications on events automatically without requiring Claude to call a tool.