feat(googlechat): full [googlechat] section — credentials + connection config-first

[chaodu-agent]

Summary

Extend [googlechat] beyond the trust fields (#1366): graduate from shared PlatformTrustConfig to a dedicated struct. Config-first invariant: [googlechat] → GOOGLE_CHAT_* env → default.

Fields to add

enabled, sa_key_file, sa_key_json, access_token, audience, webhook_path (env fallbacks: GOOGLE_CHAT_*)

Tasks

• Dedicated GoogleChatConfig + resolve; keep existing trust fields
• Bridge + AppState::apply_googlechat_config(); audience from config must feed the feat(gateway): warn when webhook L1 auth is unenforceable (#1356 Phase 1) #1373 L1 warning (JWT verifier construction)
• docs/google-chat.md + config-reference.md
• Regression: env-only unchanged

Refs

Umbrella: #1375 · Trust slice: #1366 · Reference: TelegramConfig (#1297) · L1: #1373

[github-actions]

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