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.

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

coderd/database/modelmethods.go

@@ -176,7 +176,7 @@ func (t Task) RBACObject() rbac.Object {
 }
 
 func (c Chat) RBACObject() rbac.Object {
-	return rbac.ResourceChat.WithID(c.ID).WithOwner(c.OwnerID.String())
+	return rbac.ResourceChat.WithID(c.ID).WithOwner(c.OwnerID.String()).InOrg(c.OrganizationID)
 }
 
 func (r GetChatsRow) RBACObject() rbac.Object {