feat: add pi-coding-agent integration

[bytes032]

Entire-Checkpoint: ca481ffde7d7

Summary

This PR adds Pi coding agent integration to Entire and aligns it with the existing hook/session/checkpoint model used by the CLI.

Included

• Pi agent registration and CLI wiring
• Managed Pi extension scaffold at .pi/extensions/entire/index.ts
• Hook bridge from Pi extension events to entire hooks pi ...
• Pi hook handlers for:
  • session-start
  • session-end
  • user-prompt-submit
  • before-tool
  • after-tool
  • stop
• Pi lifecycle event coverage in the scaffold for:
  • session_start
  • session_switch
  • session_fork
  • session_tree
  • before_agent_start
  • tool_call
  • tool_result
  • agent_end
  • session_shutdown
• Tree-aware Pi transcript parsing (id / parentId) with active branch resolution
• Leaf propagation and persistence for correct post-hoc condensation
• Active-branch prompt extraction, modified-file extraction, token usage, and checkpoint lookup
• Condensation/summarization normalization support for Pi transcripts
• Large-transcript safety improvements in parsing/truncation paths
• Setup/status/disable parity for Pi hook installation and cleanup
• Unit and integration test coverage for scaffold behavior, lifecycle mapping, transcript handling, and branch/leaf behavior

Prerequisites checklist (met)

• ✅ entire version works
• ✅ mise run lint passes
• ✅ mise run test passes
• ✅ New/updated tests are included
• ✅ Checkpoints: all commits in origin/main..HEAD include Entire-Checkpoint: trailers

Validation

• go test -tags=integration ./cmd/entire/cli/integration_test -run 'Pi|SetupPiHooks' -count=1 ✅

[bytes032]

@Soph @gtrrz-victor @khaong this should be ready for review