docs: document review guidance and retention behavior

[rianjs]

Closes #376.

Summary

• replace the placeholder repo-guidance.md output with real repo-guidance provenance and base-branch trust-boundary details
• add user-facing documentation for repo review guidance, reviewer-facing dossier context, and dossier/workbench retention behavior
• add prune and purge tests that explicitly verify dossier and workbench artifacts are removed with run data

Verification

• rtk go test ./internal/pipeline ./internal/cmd/datacmd ./internal/agents ./internal/datalifecycle -count=1
• rtk make lint