[P3][SDK-03] Signing, marketplace and private registry

Programme: #1
Epic: #32

## Objective
Distribute extensions/connectors/workflow packages with provenance, permission review, staged updates, revocation and private-registry support.

## Scope
- Package signing/provenance and publisher identity.
- Registry metadata: versions, compatibility, permissions, destinations, dependencies, licenses and maintenance owner.
- Install review showing requested authority/resources/data destinations.
- Update diff including capability/permission changes.
- Stable/beta/private channels and staged rollout.
- Emergency package revocation/disable list and rollback.
- Private organization/user registries with trust roots.
- Marketplace review signals for security, accessibility, maintenance and support status.

## Security/product rules
- Cryptographic signature proves package provenance, not safety.
- Any update requesting broader authority stops for explicit review.
- Revocation prevents new execution while preserving recoverable workspace state.

## Acceptance criteria
- [ ] Installation verifies signature/provenance and presents requested capabilities/destinations.
- [ ] Authority-increasing updates cannot auto-apply.
- [ ] Revoked package is prevented from starting new work and is visibly flagged.
- [ ] Rollback restores a compatible prior version when schema/state permits.
- [ ] Private registry packages follow the same manifest/conformance requirements.
- [ ] Marketplace metadata exposes maintainer/support/compatibility/security-review status.

## Dependencies
- SDK-01
- SDK-02
- SEC-05

**First phase:** P3
**Maturity target:** P6
**Owner:** platform

No files, tests, or entry points are named. Start by reviewing dependencies SDK-01, SDK-02, and SEC-05; completion is defined by the listed acceptance criteria for signing, authority review, revocation, rollback, private registries, and marketplace metadata.

Written by the indexing model from the issue text.