feat(gateway): add generic extension routing

[NWYLZW]

Summary

• add a generation-fenced public gateway extension registry with adapter and connection schemas
• materialize protected connections into one managed CLIProxyAPI process through a reusable Go C-shared bridge
• preserve exact connection routing, session transforms, secret isolation, streaming, cancellation, and disabled-route tombstones
• package native runtime resources dynamically and document the extension architecture

Verification

• npm run test:native
• npm run typecheck
• npm run check
• npm pack --dry-run
• git diff --check

Coordination

• uses stable connectionId and existing managed-connection binding semantics from Host SYNC
• no Host-private synchronization imports or additional persistent provider database
• no Host or Protocol API changes required