Add Google Calendar integration

- New src/shared/calendar.py: OAuth service builder, list_events,
  create_event, check_availability (reuses gmail-oauth secret)
- 3 new tools: get_calendar_events, create_calendar_event, check_availability
- Extended OAuth scopes to include calendar in gmail.py and token script
- Updated system prompt with calendar capabilities

Requires re-running scripts/get_gmail_token.py to grant the calendar scope
and updating the exec-aide/gmail-oauth secret with the new refresh token.

Closes #2

scripts/get_gmail_token.py

@@ -13,7 +13,10 @@ import json
 
 from google_auth_oauthlib.flow import InstalledAppFlow
 
-SCOPES = ["https://www.googleapis.com/auth/gmail.readonly"]
+SCOPES = [
+    "https://www.googleapis.com/auth/gmail.readonly",
+    "https://www.googleapis.com/auth/calendar",
+]
 
 
 def main():

src/shared/calendar.py

@@ -0,0 +1,72 @@
+import logging
+
+import google.auth.transport.requests
+from google.oauth2.credentials import Credentials
+from googleapiclient.discovery import build
+
+from shared.secrets import get_gmail_oauth, save_gmail_tokens
+
+logger = logging.getLogger(__name__)
+
+_service = None
+
+SCOPES = [
+    "https://www.googleapis.com/auth/gmail.readonly",
+    "https://www.googleapis.com/auth/calendar",
+]
+
+
+def get_calendar_service():
+    global _service
+    oauth = get_gmail_oauth()
+    creds = Credentials(
+        token=oauth.get("access_token"),
+        refresh_token=oauth["refresh_token"],
+        token_uri="https://oauth2.googleapis.com/token",
+        client_id=oauth["client_id"],
+        client_secret=oauth["client_secret"],
+        scopes=SCOPES,
+    )
+    if not creds.valid:
+        creds.refresh(google.auth.transport.requests.Request())
+        save_gmail_tokens(
+            creds.token,
+            creds.expiry.isoformat() if creds.expiry else "",
+        )
+    _service = build("calendar", "v3", credentials=creds, cache_discovery=False)
+    return _service
+
+
+def list_events(service, time_min, time_max, max_results=10):
+    result = service.events().list(
+        calendarId="primary",
+        timeMin=time_min,
+        timeMax=time_max,
+        maxResults=max_results,
+        singleEvents=True,
+        orderBy="startTime",
+    ).execute()
+    return result.get("items", [])
+
+
+def create_event(service, summary, start, end, attendees=None, description=None):
+    body = {
+        "summary": summary,
+        "start": start,
+        "end": end,
+    }
+    if description:
+        body["description"] = description
+    if attendees:
+        body["attendees"] = [{"email": a} for a in attendees]
+    return service.events().insert(calendarId="primary", body=body).execute()
+
+
+def check_availability(service, time_min, time_max):
+    body = {
+        "timeMin": time_min,
+        "timeMax": time_max,
+        "items": [{"id": "primary"}],
+    }
+    result = service.freebusy().query(body=body).execute()
+    return result["calendars"]["primary"]["busy"]

src/shared/gmail.py

@@ -13,7 +13,10 @@ logger = logging.getLogger(__name__)
 
 _service = None
 
-SCOPES = ["https://www.googleapis.com/auth/gmail.readonly"]
+SCOPES = [
+    "https://www.googleapis.com/auth/gmail.readonly",
+    "https://www.googleapis.com/auth/calendar",
+]
 
 
 def get_authenticated_service():