feat: add AgentGovernancePlugin for tool-call governance (#141)

* feat: add AgentGovernancePlugin for ADK tool-call governance

Integrates microsoft/agent-governance-toolkit (AGT) as an ADK
BasePlugin for centralized tool-call policy enforcement.

- Extends BasePlugin with async before_tool_callback
- Evaluates tool calls against YAML policy rules via agentmesh PolicyEngine
- Returns None (allow) or dict (deny/short-circuit) per ADK contract
- Fail-closed by default; opt-in fail_open=True for graceful degradation
- Structured audit logging for all policy decisions
- 11 tests covering allow/deny/fail-open/missing-dep/custom-agent-did

Signed-off-by: Imran Siddique <imran.siddique@microsoft.com>

* feat: add strict mode and async policy evaluation

Address review feedback:
- Add strict=True parameter that raises RuntimeError on policy load
  failures instead of silently skipping (for security-critical deployments)
- Offload synchronous PolicyEngine.evaluate to thread pool executor
  via asyncio.run_in_executor to avoid blocking the event loop
- Add 3 new tests (strict mode, non-strict skip, thread pool verification)
- Update sample README with strict mode documentation

Signed-off-by: Imran Siddique <imran.siddique@microsoft.com>

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Signed-off-by: Imran Siddique <imran.siddique@microsoft.com>

chore: update last-release-sha for next release

chore(release/candidate): release 0.4.0 (#76)

chore(release/candidate): release 0.3.1 (#72) (#73)

chore(release/candidate): release 0.3.0 (#65) (#66)

Update version.py

Release version 0.1.0

Release version 0.0.1