Expose IFC policy to external agent harnesses

[jmecom]

Adds buzz-ifc-broker, a bounded JSON-RPC process that exposes the shared buzz-ifc rules to non-Rust agent harnesses such as kgoose. It supports domain derivation, worker entry and retirement, input observation, call checks, publication checks, and structured decision logging over stdio.

The executable is the policy process inside a larger trusted gateway, not the complete Buzz broker. Its caller must still verify signed events and authoritative membership, control worker lifecycle and publication, and provide any required credential or OS confinement.

The protocol includes bounded request frames, stable error responses, a golden domain fixture, and stdio conformance coverage. This commit adds no new enforcement to buzz-acp; it exposes the policy already established by the lower layers.

Testing:

• cargo test -p buzz-ifc -p buzz-ifc-broker -p buzz-acp (835 tests)
• cargo clippy -p buzz-ifc -p buzz-ifc-broker -p buzz-acp --all-targets -- -D warnings
• The final tree is identical to the previously verified implementation, for which just ci passed.

Stack:

1. Audit audience-scoped IFC decisions in buzz-acp #6273: audit-only IFC evaluation
2. Route ACP workers by execution domain #6274: exact-domain ACP worker routing
3. Extract reusable IFC policy and classify worker placement #6275: reusable policy and worker placement
4. Expose IFC policy to external agent harnesses #6176: external harness broker