Remove CodeRabbit extraction command

[jordanpartridge]

Summary

Remove the coderabbit:extract command and all associated code.

Rationale

CodeRabbit extraction was an early experiment in pulling third-party review data into the knowledge base. It's being superseded by deeper, native PR analysis capabilities that will provide:

• Commit history and diff analysis
• PR timeline and activity tracking
• Comment thread summarization
• Merge readiness signals (approvals, CI status, conflicts)
• Reviewer sentiment analysis

All sourced directly from the GitHub API via gh — no third-party dependency required.

Scope

• Remove coderabbit:extract command class
• Remove any associated tests
• Remove any CodeRabbit-specific models, services, or config
• Clean up any references in documentation

Replaces

PR #117 (feat/coderabbit-extract-89) which originally added this functionality.

[jordanpartridge]

Done in #126 (29b132f)