Document safe migration operations and contributor input boundaries #57

Blocked by: none. Parent specification: #54. Task T3.

Document runnable operator migration setup, trusted config, explicit phase/pattern/dry-run/allowlist, batch-only DUAL_WRITE, selected-key vs scan budget, no checkpoint, reserved sidecar namespace and overwrite/rollback/concurrency limitations. Preserve runtime behavior and existing PR #53. Add public bug-template redaction reminders and root .gitignore whitelist contributor guidance. Update canonical owners only; no new operator platform or runtime switches.