feat: add Calendly integration

[AggManik]

Summary

• Adds Calendly integration with OAuth2 auth
  (https://auth.calendly.com/oauth/authorize)
• Implements 31 MCP tools across identity, event types, scheduled events,
  invitees, scheduling links, webhooks, routing forms, groups, org invitations,
  and availability
• Webhook activity ingestion for invitee.created, invitee.canceled,
  routing_form_submission.created
• All list tools have explicit count + page_token cursor pagination and
  ISO time filters where applicable

Tools implemented

Category	Tools
Identity + Org	get_current_user, get_user, get_organization,
list_organization_memberships
Event Types	list_event_types, get_event_type,
get_event_type_availability, list_event_type_available_times
Scheduled Events	list_scheduled_events, get_scheduled_event,
cancel_scheduled_event
Invitees	list_event_invitees, get_event_invitee
Scheduling Links	create_scheduling_link,
create_single_use_scheduling_link
Webhooks	create_webhook_subscription, list_webhook_subscriptions,
get_webhook_subscription, delete_webhook_subscription
Routing Forms	list_routing_forms, get_routing_form,
get_routing_form_submission
Groups	list_groups, get_group
Org Invitations	list_organization_invitations,
create_organization_invitation, revoke_organization_invitation
Availability	list_user_availability_schedules,
get_user_availability_schedule, list_user_busy_times