feat: add organization scoping to chats (#23827)

Fixes https://github.com/coder/internal/issues/1436

* Adds organization_id to chats with backfill (workspace org → user org membership → default org)
* No support yet for ACLs (follow-up issue)
- Cross-org workspace binding rejected (both in `CreateChatRequest` and in `create_workspace` tool
- Adds `OrganizationAutocomplete` to `AgentCreateForm`
- Docs updated with `organization_id` in chats-api.md

> 🤖 Written by a Coder Agent. Reviewed by many humans and many agents.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

coderd/database/migrations/000467_chat_organization_id.down.sql

@@ -0,0 +1 @@
+ALTER TABLE chats DROP COLUMN organization_id;

coderd/database/migrations/000467_chat_organization_id.up.sql

@@ -0,0 +1,20 @@
+-- Step 1: Add nullable column with FK.
+ALTER TABLE chats
+    ADD COLUMN organization_id UUID REFERENCES organizations(id) ON DELETE CASCADE;
+
+-- Step 2: Backfill from workspace org (primary path). Fall back to
+-- user's oldest org membership, then default org for rows where
+-- workspace_id was NULLed out by ON DELETE SET NULL or never set.
+UPDATE chats c
+SET organization_id = COALESCE(
+    (SELECT w.organization_id FROM workspaces w WHERE w.id = c.workspace_id),
+    (SELECT om.organization_id FROM organization_members om
+     WHERE om.user_id = c.owner_id ORDER BY om.created_at ASC LIMIT 1),
+    (SELECT id FROM organizations WHERE is_default = true LIMIT 1)
+);
+
+-- Step 3: Enforce NOT NULL going forward.
+ALTER TABLE chats ALTER COLUMN organization_id SET NOT NULL;
+
+-- Step 4: Index for efficient lookups by organization.
+CREATE INDEX idx_chats_organization_id ON chats (organization_id);