pi-claude-code-provider fences off Claude Code's MCP servers by design,
so managed MCPs like Google Calendar can't reach the bot through that
provider. Implement calendar access as a first-class Pi tool instead —
it lives alongside `schedule` and calls Google Calendar API v3 directly
with a subscription-free refresh-token grant.
Config: three new optional env vars (GOOGLE_CLIENT_ID,
GOOGLE_CLIENT_SECRET, GOOGLE_REFRESH_TOKEN) — set all three to enable
the tool, leave all three blank to disable. Partial config raises a
ConfigError at startup so mis-set env is caught early.
Tool: list / create / update / delete on any calendar (default
'primary'). Access tokens are cached until expiry and refreshed on
demand from the long-lived refresh token. Invalid_grant surfaces a
clear "re-run device flow" error.
