Integrate Google calendar

[aniketmaurya]

This pull request adds Google Calendar integration to the application, alongside several improvements to environment configuration and agent setup. The main changes include a new CalendarTool for interacting with Google Calendar, updates to the agent context and credential management to support calendar access, and the addition of new function tools for listing and retrieving calendar events.

Google Calendar Integration

• Added a new CalendarTool class to provide read-only access to Google Calendar, including methods for listing events and retrieving a single event. (src/zerozen/integrations/google/calendar_tool.py)
• Introduced new function tools: list_calendar_events and get_calendar_event, enabling the agent to interact with calendar data. (src/zerozen/integrations/google/agent.py)
• Updated the AppContext and agent construction to include the calendar tool and credentials. (src/zerozen/integrations/google/agent.py) [1] [2]
• Extended the module’s exports to include CalendarTool. (src/zerozen/integrations/google/__init__.py)

Credential Management

• Added a new credential provider factory, desktop_calendar_creds_provider_factory, to handle Google Calendar authentication separately from Gmail. (src/zerozen/integrations/google/creds.py) [1] [2]

Agent and Prompt Updates

• Updated the system prompt and agent naming to reflect support for both Gmail and Calendar, and improved instructions for calendar-related queries. (src/zerozen/integrations/google/agent.py) [1] [2]
• Changed import to use the new unified Google agent in the main agent hub. (src/zerozen/agenthub/main.py)

Environment Configuration

• Added support for loading environment variables from a .env file at startup. (src/zerozen/chat.py)