Add read-only Google Calendar integration via gws CLI

Adds a get_calendar_events agent tool that shells out to `gws calendar +agenda`
for admin users. Controlled by GOOGLE_CALENDAR_ENABLED env var, with OAuth
credentials mounted from credentials.json.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -8,7 +8,7 @@ RUN apt-get update && apt-get install -y \
     curl \
     && curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
     && apt-get install -y nodejs \
-    && npm install -g yarn obsidian-headless \
+    && npm install -g yarn obsidian-headless @anthropic/gws \
     && rm -rf /var/lib/apt/lists/* \
     && curl -LsSf https://astral.sh/uv/install.sh | sh