feat(sync): bridge remote connect into polling engine

[fujibee]

Summary

• load per-team remote binding and 0600 credential artifacts directly in the polling engine
• remove ambient bearer-token authentication and keep private credentials out of argv, logs, and driver children
• align pairing exchange and self-revoke requests with the v1 protocol/team binding headers
• validate the complete canonical capability snapshot before committing onboarding state
• exercise real remote connect, two device credentials, message/read sync, retention resync, and revoke in the PostgreSQL E2E

Compatibility

• plaintext-capable bindings need no separate sync configure step
• age-v1 keeps the explicit authenticated snapshot/checkpoint configure path; the engine refuses to synthesize authority state from the simpler onboarding key record
• branch remains Draft/unmerged for dogfood

Verification

• remote onboarding Bats: 38 tests
• key Bats: 19 tests
• remote sync engine Node: 25 tests
• JSONL/SQLite/engine Bats: 41 tests
• PostgreSQL server/integration: 17 tests
• server typecheck and build