Add agenttrace package for Agent Trace schema support

[TanGentleman]

Summary

• Adds agenttrace/ package implementing the Agent Trace v0.1.0 open specification for tracing AI-generated code
• Provides Go types (TraceRecord, File, Conversation, Range, Contributor, Tool, Vcs) matching the JSON schema exactly
• Includes a thread-safe Recorder for accumulating file edits from hookshot handlers and building trace records
• Adds WriteTrace/ReadTrace for JSON file I/O (stdlib only, zero new dependencies)
• Updates the multi-hook example to demonstrate integration with OnAfterFileEdit

Test plan

• go build ./... compiles all packages
• go test ./... passes (types round-trip, recorder workflow, store I/O)
• go vet ./... clean

[corridor-security]

Summary: This PR adds an agenttrace package for recording and (optionally) storing JSON trace records and updates the example hook to record edited file paths for attribution; no new command execution, network calls, or privileged operations are introduced.

Risk: Low risk. The changes operate on in-memory data and file I/O only, with the example merely recording file paths without writing or executing them, and no new externally exposed surfaces are added.