feat: restrict task invocation by caller

[superironpanda]

Summary

• add optional allowed_surfaces and allowed_requesters task frontmatter
• add WithSurface and structured WithCaller invocation options
• reject disallowed invocations before task content is assembled
• add -surface, -caller, and -caller-email CLI flags
• keep linting caller-independent while still parsing the new fields

Behavior

• omitted or empty allowlists preserve existing unrestricted behavior
• values within each allowlist use case-insensitive OR matching
• when both allowlists are configured, both restrictions must match
• requester matching accepts either username or email
• authorization failures return ErrCallerNotAllowed, including for lenient search paths

Documentation

Updates the specification, task-authoring guide, and CLI reference.