feat(plugins): add AuthorityRoutingPlugin — pre-execution authority posture (ADVISE/EXECUTE/DEFER/STOP) #171

Problem

Tool schemas and permissions govern access — which tools exist and who may call them. They do not govern authorization: whether the agent was allowed to act at all for a given request, and within what scope. That gap is where three recurring agent failures live:

• advice becomes action — the user asked for a recommendation and the agent executed anyway;
• scope expansion — one bounded edit ballooned into a dependent workflow;
• missing approval — an approval was clearly required and the agent proceeded.

AgentGovernancePlugin (policy-as-code allow/deny) and the HITL approval gateway answer "is this tool call permitted by policy?". Neither answers "was the agent authorized to act at all, and within what scope?" — and both require external infrastructure (a policy engine / an approval service).

Proposal

Add AuthorityRoutingPlugin, a self-contained BasePlugin that assigns an authority posture to every tool call before it runs:

Two stages produce the final posture:

1. Posture router (optional, caller-supplied callable — typically a model call) returning a validated verdict with a scope statement.
2. Deterministic guard — an irreversibility keyword tripwire + an approval-state check — composed with the router under most-restrictive-wins: plain code can only make a posture stricter, never looser.

Design properties:

• Fail-closed — an unparseable or raised router verdict defaults to DEFER.
• Guard-only mode (no router) is fully deterministic and needs no model and no extra dependency.
• Complements, rather than duplicates, the existing governance plugins.

Scope

• src/google/adk_community/plugins/authority_routing_plugin.py + export
• tests/plugins/test_authority_routing_plugin.py (deterministic, no credentials)
• contributing/samples/authority_routing/ (runnable sample with live output)

Happy to adjust the API surface or placement to match maintainer preferences. I have an implementation ready and will open a PR referencing this issue.