Implement Policy Fabric admission and denial events

[mdheller]

Agent work order

@copilot @codex please implement this after PR #1 lands or by branching from seed/agentterm-chatops-core if instructed.

Goal

Add a Policy Fabric admission layer before AgentTerm dispatches side-effecting commands or releases sensitive context.

Context

AgentTerm is the operator surface. Policy Fabric remains the decision/evidence authority for side effects and sensitive context release.

Required scope

• Add a policy admission interface behind the adapter boundary.
• Evaluate at least these event classes: shell session, workspace materialization, workroom context hydration, memory recall/writeback, semantic membrane, Holmes investigation request, Sherlock Search hydration, MeshRush graph operation, AgentPlane run, GitHub mutation, CI retry.
• Emit explicit policy_check, decision, and denial events into EventStore.
• Add tests for allow/deny/pending states using a fake policy backend.
• Keep the initial implementation local/stubbed; do not require a live Policy Fabric service in CI.

Non-goals

• Do not move Policy Fabric schemas into this repo.
• Do not silently execute side effects when admission fails or is unknown.

Validation

ruff check .
pytest