Webhook endpoint for Linear events.

Handles:

• OAuthAuthorization revoked events: When Deeptrace integration is revoked, removes linear_oauth_token_id and linear-mcp from team config.
• Issue create/update events: Triggers investigations when configured (feature-flagged).
• AgentSessionEvent: Chat-style responses when @Deeptrace is mentioned, using Linear's Agent Interaction protocol (AgentActivity mutations).
• Comment events: Ignored (agent mentions use AgentSessionEvent instead).

Linear webhook payload structure: { "action": "create" | "update" | "revoked" | "created" | "prompted", "type": "Issue" | "Comment" | "OAuthAuthorization" | "AgentSessionEvent", "organizationId": "...", "data": { ... } // for Issue/Comment "agentSession": { ... } // for AgentSessionEvent }