feat(plugin): add pull-replicator scaffold for external sync (#72)

[CuboYe]

/claim #72

This PR delivers a concrete first implementation slice for issue #72 by adding a replication control-plane plugin scaffold.

Implemented

• New plugin: plugins/pull-replicator
• Creates replication control tables automatically:
  • tmp_replication_jobs
  • tmp_replication_runs
• Registers health endpoint:
  • GET /api/plugins/pull-replicator/health
• Wires plugin into runtime in src/index.ts
• Includes unit test for table bootstrap behavior

Why this PR-first scope

Issue #72 is broad and has a history of abandoned attempts. This PR intentionally lands a reviewable, production-usable foundation (schema + lifecycle wiring) that follow-up PRs can extend with connector adapters, checkpointed pull execution, and upsert conflict strategies.

Follow-up slices

1. Adapter interface + concrete source connectors
2. Checkpointed incremental pull execution
3. Admin trigger/status/retry endpoints + demo video