Add SidClaw Governance Service integration

## Summary

Add a governance service integration for Google ADK agents using [SidClaw](https://sidclaw.com) — policy evaluation, human-in-the-loop approval, and tamper-proof audit trails for tool calls.

## Background

This was invited by @rohityan in [google/adk-python#5081](https://github.com/google/adk-python/pull/5081):

> "Hi @VladUZH, Closing this PR here as it belongs to adk-python-community repo. We highly recommend releasing the feature as a standalone package that we will then share through: https://google.github.io/adk-docs/integrations/"

## What It Does

The `SidClawGovernanceService` integrates with ADK's `before_tool_callback` and `after_tool_callback` hooks:

1. **Before tool execution** — evaluates the action against SidClaw policies (allow / deny / require approval)
2. **After tool execution** — records the outcome in SidClaw's hash-chain audit trail

## Implementation

- New module: `src/google/adk_community/governance/`
- Uses the `sidclaw` PyPI package (Apache 2.0, published)
- Async-only (matches ADK callback pattern)
- Configurable per-tool data classifications and approval behavior
- Fail-closed by default when SidClaw is unreachable
- Comprehensive unit tests (20 test cases)

## CLA

Already signed — see [google/adk-python#5081](https://github.com/google/adk-python/pull/5081).

Bắt đầu trong src/google/adk_community/governance/ và xem xét các điểm tích hợp before_tool_callback và after_tool_callback của ADK. Sử dụng package sidclaw để bao quát các quyết định về policy, phê duyệt và ghi audit, sau đó xác minh các phân loại có thể cấu hình, hành vi fail-closed và 20 trường hợp unit-test đã nêu.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.