Add Claude Code extension and tune Spy compaction detection

[jimpudar]

Summary

• Add a built-in claude-code Rootcell extension that installs Claude Code with Bedrock-oriented defaults.
• Extend Spy to normalize Claude Code Bedrock traffic, including Anthropic Messages invoke and invoke-with-response-stream paths.
• Add compaction heuristics for Claude Code summary-style requests and surface a dedicated UI/API label and reason set.
• Update fixtures, tests, docs, proxy allowlists, and Nix evaluation coverage for the new extension and detector behavior.

Testing

• Added and updated unit tests for Rootcell extension registration, Bedrock normalization, and compaction detection.
• Ran Spy typecheck and lint successfully.
• Validated the full test suite, including service-level tests and fixture-backed Bedrock/compaction coverage.
• Performed local provisioning and end-to-end Spy verification against captured Claude Code Bedrock traffic.