feat(mcp): attach existing webhooks to agents (PRO-1951)

[prat11m]

Summary

Follow-up to #32 (PRO-1950). That pass only exposed org-level /webhook CRUD, which forced "create + attach" every time and let the model assume event types. The backend actually has an agent-scoped subscription API to attach an already-created webhook to an agent — this PR wraps it.

New tools

Tool	Route
attach_agent_webhook	POST /agent/:id/webhook-subscriptions {webhookId, eventTypes[]}
get_agent_webhooks	GET /agent/:id/webhook-subscriptions
detach_agent_webhooks	DELETE /agent/:id/webhook-subscriptions

Behavior fixes (the actual asks)

• Attach from the existing list, don't recreate: attach_agent_webhook points an agent at an existing webhook. create_webhook's description now steers callers to use it (and to get_webhooks first) instead of registering a duplicate.
• Never assume events: event_types is required on attach, and every event field carries an explicit "do not guess — ask the user" instruction.
• create_webhook.events is now optional (backend accepts an empty array) so you can register an endpoint-only webhook, then attach separately.

Important backend semantics (documented in the tool)

POST /agent/:id/webhook-subscriptions replaces the agent's subscriptions — it deletes all existing subs for the agent, then recreates them for the given webhook only. So an agent points at one webhook at a time via this endpoint. (agent.controller.ts:972-996)

Flow

get_webhooks (pick from the list) → attach_agent_webhook (with the events the user chose).

Test plan

• get_webhooks lists existing endpoints.
• attach_agent_webhook with an existing webhook_id + explicit event_types → agent receives those events; verify via get_agent_webhooks.
• Re-attaching a different webhook replaces the prior subscriptions (not additive).
• detach_agent_webhooks clears them.
• create_webhook without events registers an endpoint-only webhook.

Linear

PRO-1951

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