Relates to CODAGT-713

Depends on #27284

This makes the per-template `agents_allowed` field authoritative in the API and chatd. It adds optional create and metadata update fields with the intended default and omission semantics, supports `agents-allowed:` template search, includes the value in telemetry, and makes `list_templates`, `read_template`, and `create_workspace` read the template row directly. Existing-workspace retries remain idempotent, and blocked same-organisation templates return an actionable message.

The experimental `/template-allowlist` routes remain temporarily because the shipped AI Settings page still calls them, but they no longer control chatd enforcement. #27514 moves that page to per-template metadata, #27515 removes the legacy storage, routes, SDK types, and utility, #27517 adds the CLI flags, and #27518 updates the platform controls documentation for the per-template model, directly addressing CRF-5 and CRF-6. The stack is intended to merge as a unit.