integrations: add Attenu Guard plugin (per-agent permissions on tool calls and transfers)

[rafaelasor]

Adds docs/integrations/attenu-guard.md (+ catalog icon) for the Attenu Guard ADK plugin (Apache-2.0, PyPI attenu-guard, extra attenu-guard[google-adk]).

The plugin is one BasePlugin (attenu_guard.adapters.google_adk): before_agent_callback computes each sub-agent's permission set as the meet of the parent's and the declared child set when control reaches it (covers transfer_to_agent, AgentTool and task-mode sub-agents), and before_tool_callback checks each tool call before the tool body runs. Decisions land in a hash-chained audit log verified offline. Tested against google-adk 2.7.1; the repo ships a scripted-model example (no API key) and a live smoke test.

Follows the plugin-page conventions (frontmatter, language tag, Use cases, Prerequisites, Installation, Use with agent, Resources). Disclosure: I maintain attenu-guard. I have signed / will sign the Google CLA on this PR.

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[rafaelasor]

@googlebot I signed it!