Route Google Chat mentions and slash commands to separate handlers #1

Context

The Google Chat webhook (GH-2) only handles @mentions (chat.messagePayload). Slash commands arrive as chat.appCommandPayload (captured in tests/fixtures/google-slash-command.json) and are dropped at the endpoint. We want both event kinds to reach the worker and be handled separately, as groundwork for ingesting /add-knowledge content later.

Decisions

• handle_event only decides the event kind and delegates.
• Private _handle_mention and _handle_command in app/services/google_chat.py each do their own logging (no ingestion yet).
• Model appCommandPayload in app/schemas/google_chat.py (shares space/message shape with messagePayload, adds appCommandMetadata).
• The endpoint dispatches and acks slash commands too; only events with neither payload still get {}.

Scope

• Schema: AppCommandMetadata, AppCommandPayload, ChatPayload.app_command_payload
• Service: dispatch + two private handlers
• Router: stop dropping slash commands
• Tests driven by both fixtures

Out of scope

• Actually ingesting the /add-knowledge text into the RAG pipeline
• Routing between multiple slash commands

Builds on GH-2.

Full design doc: docs/plans/006-google-chat-mention-command-handlers.md