MCP Events add session webhooks and triggers on MCP server events

Organizations that turn on the experimental MCP Events flag get events in both directions.

• An MCP client calls events/subscribe on /mcp to receive signed webhooks for session.completed, session.failed and session.input_required. Callbacks must be public HTTPS URLs and are verified before the first event.
• An agent trigger with trigger_type: "mcp_event" subscribes to an event one of the agent’s own MCP servers publishes, such as a tracker announcing new issues, and starts a session when it arrives.

See MCP and Agent triggers. PRs #3931, #3970.