feat(agent-control): add P0 registry and source discovery

[1012839419a-alt]

Parent contract: #140
Program: #130
Source dependency: #141

Dependency

Do not implement until #140 is approved. This is slice A and gates every later Agent-data slice.

Owned paths

• backend/agent_control/schemas.py
• backend/agent_control/registry.py
• backend/agent_control/router.py
• backend/agent_control/source_projection.py
• backend/api/v1/agent_control.py
• tests/unit/test_agent_control_registry.py
• tests/integration/test_agent_control_discovery_api.py

The coordinator alone owns backend/agent_control/builtin_registry.py, backend/api/v1/__init__.py, and integration conflict resolution.

Required behavior

Implement the v1 envelope/result/ref/gap/error contracts, authoritative registry, capabilities.list, and source.readiness. Project #141 capability intelligence.source.aihot through the generic Source projection; do not add an aihot.* command or provider-specific MCP/Chat tool. Preserve the five accepted workflow.* registry entries and fail closed on Project/RBAC/readiness boundaries.

Acceptance gate

Registry and schema discovery are deterministic and transport-neutral; availability and missing-plugin/source reasons are truthful; wrong Project or RBAC is denied without data; AIHOT usage authorization is an explicit readiness condition. Targeted unit and API integration tests pass before slice B starts.

[1012839419a-alt]

Implementation candidate committed on dedicated worktree: 53f38693. It adds strict v1 envelope/result/ref/gap/error schemas; a deterministic multi-version registry containing seven P0 plus five workflow compatibility identifiers; transport-neutral CapabilityRouter handlers for capabilities.list and source.readiness; generic installed-source registration/projection; exact Workspace/Project/RBAC and expected-revision gates; explicit AIHOT domain/usage readiness; and an unregistered-command fail-closed path. Fresh evidence: 10 slice tests passed, 34 slice+existing Agent Data/MCP regressions passed, Ruff clean, mypy clean, git diff --check clean. Production coordinator glue (builtin_registry, API v1 inclusion, AIHOT registration) is intentionally not in this child commit and remains required before closure. Independent review is queued; do not close yet.