EPIC: Extensions, SDKs and ecosystem

Programme: #1

Own the sandboxed extension host, connector/agent/UI/workflow SDKs, simulator/devtools, signing, marketplace governance and private registries.

## Child issues
- [ ] #98 SDK-01 — Sandboxed extension host and manifests
- [ ] #99 SDK-02 — SDKs, simulator and developer tools
- [ ] #100 SDK-03 — Signing, marketplace and private registry

## Cross-cutting gates
Permission manifests are explicit, local fixtures support development without broad live credentials, authority increases require review, package provenance/update channels are verifiable, and revoked packages cannot execute new work.

Start with child issues #98, #99, and #100 to understand the separate workstreams: the sandboxed extension host and manifests, SDKs and simulator/devtools, and signing, marketplace, and private registry work. Done requires explicit permission manifests, local fixtures without broad live credentials, review for authority increases, verifiable provenance and update channels, and revoked packages being unable to execute new work.

Written by the indexing model from the issue text.