Add a session concentration investigation demo

[kwisatzh]

Summary

• Add sh examples/investigate.sh --sessions, an offline example that works with the released v0.3.0 binary and requires no Docker or API key.
• Show a synthetic session accounting for 3,000 of 3,300 tokens, with bounds and a conservative runaway-candidate flag. Document that this is not proof of an agent loop.
• Check in reproducible summary fixtures and their generator, privacy assertions, report tests, and packaged-binary smoke coverage.
• Link the demo from the README and explain the distinction between synthetic summaries and live collector evidence.

Verification

• go test -race ./...
• go vet ./...
• Packaged-binary smoke test against the published v0.3.0 darwin/arm64 binary
• All four investigation demo modes
• git diff --check and public-file privacy scan

No new release is needed for this example.