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/models.go

@@ -4246,6 +4246,7 @@ type Chat struct {
 	LastReadMessageID   sql.NullInt64         `db:"last_read_message_id" json:"last_read_message_id"`
 	LastInjectedContext pqtype.NullRawMessage `db:"last_injected_context" json:"last_injected_context"`
 	DynamicTools        pqtype.NullRawMessage `db:"dynamic_tools" json:"dynamic_tools"`
+	OrganizationID      uuid.UUID             `db:"organization_id" json:"organization_id"`
 }
 
 type ChatDiffStatus struct {