feat: add session investigation evidence bundle

[jmagar]

Summary

• adds a session-rooted investigation/evidence bundle
• correlates transcript, graph neighborhood, Agent Observatory repo/worktree/commit evidence, skills/MCP/hooks, artifacts, OTLP, incidents, notifications, related sessions, and retention lineage
• preserves bounded/truncation semantics and explicit trust for transcript-extracted external references

Validation

• cargo fmt --check
• git diff --check
• focused session investigation tests
• ai session graph-correlation tests
• MCP typed-request/schema dispatch tests
• cargo clippy --all-targets --locked -- -D warnings

This PR will receive additional follow-up slices before merge.

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