feat(chatd): add start_workspace tool to agent flow (#22646)

## Summary

When a chat's workspace is stopped, the LLM previously had no way to
start it — `create_workspace` would either create a duplicate workspace
or fail. This adds a dedicated `start_workspace` tool to the agent flow.

## Changes

### New: `start_workspace` tool
(`coderd/chatd/chattool/startworkspace.go`)
- Detects if the chat's workspace is stopped and starts it via a new
build with `transition=start`
- Reuses the existing `waitForBuild` and `waitForAgent` helpers (shared
logic)
- Shares the workspace mutex with `create_workspace` to prevent races
- Idempotent: returns immediately if the workspace is already running or
building
- Returns a `no_agent` / `not_ready` status if the agent isn't available
yet (non-fatal)

### Updated: `create_workspace` stopped-workspace hint
- `checkExistingWorkspace` now returns a `stopped` status with message
`"use start_workspace to start it"` when it detects the chat's workspace
is stopped, instead of falling through to create a new workspace

### Wiring
- `chatd.Config` / `chatd.Server`: new `StartWorkspace` /
`startWorkspaceFn` field
- `coderd/chats.go`: new `chatStartWorkspace` method that calls
`postWorkspaceBuildsInternal` with proper RBAC context
- `coderd/coderd.go`: passes `chatStartWorkspace` into chatd config
- Tool registered alongside `create_workspace` for root chats only (not
subagents)

### Tests (`startworkspace_test.go`)
- `NoWorkspace`: error when chat has no workspace
- `AlreadyRunning`: idempotent return for workspace with successful
start build
- `StoppedWorkspace`: verifies StartFn is called, build is waited on,
and success response returned

coderd/chatd/chatd.go

@@ -66,6 +66,7 @@ type Server struct {
 
 	agentConnFn       AgentConnFunc
 	createWorkspaceFn chattool.CreateWorkspaceFn
+	startWorkspaceFn  chattool.StartWorkspaceFn
 	pubsub            pubsub.Pubsub
 	webpushDispatcher webpush.Dispatcher
 	providerAPIKeys   chatprovider.ProviderAPIKeys
@@ -852,6 +853,7 @@ type Config struct {
 	InFlightChatStaleAfter     time.Duration
 	AgentConn                  AgentConnFunc
 	CreateWorkspace            chattool.CreateWorkspaceFn
+	StartWorkspace             chattool.StartWorkspaceFn
 	Pubsub                     pubsub.Pubsub
 	ProviderAPIKeys            chatprovider.ProviderAPIKeys
 	WebpushDispatcher          webpush.Dispatcher
@@ -887,6 +889,7 @@ func New(cfg Config) *Server {
 		subscribeFn:                cfg.SubscribeFn,
 		agentConnFn:                cfg.AgentConn,
 		createWorkspaceFn:          cfg.CreateWorkspace,
+		startWorkspaceFn:           cfg.StartWorkspace,
 		pubsub:                     cfg.Pubsub,
 		webpushDispatcher:          cfg.WebpushDispatcher,
 		providerAPIKeys:            cfg.ProviderAPIKeys,
@@ -2210,6 +2213,14 @@ func (p *Server) runChat(
 				AgentConnFn: chattool.AgentConnFunc(p.agentConnFn),
 				WorkspaceMu: &workspaceMu,
 			}),
+			chattool.StartWorkspace(chattool.StartWorkspaceOptions{
+				DB:          p.db,
+				OwnerID:     chat.OwnerID,
+				ChatID:      chat.ID,
+				StartFn:     p.startWorkspaceFn,
+				AgentConnFn: chattool.AgentConnFunc(p.agentConnFn),
+				WorkspaceMu: &workspaceMu,
+			}),
 		)
 		tools = append(tools, p.subagentTools(func() database.Chat {
 			return chat