Add config-backed pipeline runtime

[mdheller]

Summary

Adds JSON configuration loading for the AgentTerm dispatch pipeline and wires it into agent-term-dispatch.

It adds:

• config.py with typed runtime config models.
• Parsing for eventStore, matrix, agentRegistration, planes, participants, and local runtime fixtures.
• AgentTermConfig.pipeline_config() so Matrix posture and Agent Registry switches flow into the dispatch pipeline.
• --config support for agent-term-dispatch.
• Config-driven event store path, participant agent IDs, local registered agents, local tool grants, and local policy fixture decisions.
• Tests for config parsing, safe defaults, local runtime fixtures, pipeline posture switches, and config-backed dispatch.

Boundary decisions

• Config is declarative. It can express desired local bindings and test fixtures, but does not become authority for agents, policy, Matrix, or SourceOS planes.
• Agent Registry remains the authority for identity, sessions, grants, revocation, and runtime authority.
• Policy Fabric remains the authority for admission.
• Matrix E2EE posture remains enforced by the pipeline config.
• Live service-backed adapters are still a separate lane.

Validation

Expected CI:

ruff check .
pytest

Next lane after merge: replace in-memory fixtures with first live service-backed adapter behind the same config boundary.