v0.1.0 — first tagged release

The public developer surface for 42min:

- plugins/42min: Cursor plugin, manifests validated against Cursor's own
  published schemas in CI. Marketplace submission pending; installs
  locally today.
- openapi/42min.v1.yaml: OpenAPI 3.1 for the whole /v1 surface and the
  eight webhook events, checked against the running API by a contract
  suite. Also served at https://api.42min.us/openapi.yaml.
- mcp/tools.json: the MCP tool contract, 35 tools generated from the live
  registry and drift-gated.
- examples/webhook-receiver: a dependency-free receiver that verifies
  signatures, survives malformed input, and recovers after a crash.