Emit durable migration manifests and support idempotent reruns #74

Idea

Make backlog migrations emit a durable machine-readable manifest and support safe reruns.

Echo's migration produced a useful Markdown archive README and a temporary local JSON file, but the durable repo artifact is not machine-readable enough for future tooling. Reruns should also be idempotent rather than risking duplicate issues.

Possible shape

• Commit docs/method/graveyard/github-issue-migration/manifest.json beside the human README.
• Include original path, archived path, issue URL/number, source lane, final labels, original file digest, migration tool version, migration timestamp, and repo remote.
• Stamp migrated issues with a hidden marker or stable footer containing the source path and digest.
• On rerun, detect existing migrated issues by marker before creating anything.
• Provide --dry-run, --resume, and --verify-manifest modes.

Why it matters

The migration should be auditable by tools, not only humans reading a large table. Idempotency also makes the script safe for real repositories with partial failures.

Acceptance sketch

• A failed migration can be resumed without duplicate issues.
• A completed migration can be verified against GitHub Issues and archive files.
• The manifest is deterministic except for explicit timestamp/run metadata.
• Lane/label preservation bugs become easier to detect automatically.