fix(agent/agentproc): track the default working directory for chat processes (#30674)

A process started through the agent's process API without a `workdir`
runs in the agent's default directory, but only an explicit `workdir`
was recorded in the chat's path store. Repositories touched that way
were never watched by the git panel.

The path store now records `proc.workDir`, the directory the process
actually ran in, whether requested or defaulted.

### Before


https://github.com/user-attachments/assets/b98c290a-0c77-4ce4-bb00-eb4088d2067e

### After


https://github.com/user-attachments/assets/0383aa1a-eced-4338-97af-3c54501cdedb

<details>
<summary>Decision log</summary>

The git panel's refresh button should show everything a page reload
would. Investigation found five ways it didn't, each fixed in its own PR
on the current `changes` protocol:

1. `execute` without a workdir didn't record the directory the process
ran in (`agent/agentproc`).
2. Refresh only rescanned repos already being watched, so a repo created
or recreated after its path was recorded stayed hidden
(`agent/agentgit`). A connection with no repos also got no first
message.
3. After a git watch reconnect, the new full state was merged into the
previous connection's map, so deleted repos stayed visible
(`useGitWatcher`).
4. A chat watch reconnect didn't refetch the open chat, so missed
`diff_status_change` events were lost (`AgentsPageLayout`).
5. Refresh didn't refetch the chat, whose diff statuses back the PR
tabs, and was disabled until git context existed (`GitPanel`,
`AgentChatPageView`).

Considered and deferred: a snapshot protocol with diffs fetched by hash,
which would keep large multi-repo diffs under the 4 MiB proxy read
limit, and making scans faster at scale. Neither is needed to fix
refresh, and together they made a single ~5900-line change that couldn't
be reviewed. The refresh spinner still runs for a fixed time, since
tying it to the response needs a request ID in the protocol.

</details>

---

_Generated by Coder Agents on behalf of @DanielleMaywood._

agent/agentproc/api.go

@@ -106,14 +106,15 @@ func (api *API) handleStartProcess(rw http.ResponseWriter, r *http.Request) {
 
 	// Notify git watchers after the process finishes so that
 	// file changes made by the command are visible in the scan.
-	// If a workdir is provided, track it as a path as well.
+	// The directory the process ran in counts as touched, whether
+	// it was requested or defaulted.
 	if api.pathStore != nil {
 		if chatContext, ok := agentchat.FromContext(ctx); ok {
 			allIDs := append([]uuid.UUID{chatContext.ID}, chatContext.AncestorIDs...)
 			go func() {
 				<-proc.done
-				if req.WorkDir != "" {
-					api.pathStore.AddPaths(allIDs, []string{req.WorkDir})
+				if proc.workDir != "" {
+					api.pathStore.AddPaths(allIDs, []string{proc.workDir})
 				} else {
 					api.pathStore.Notify(allIDs)
 				}

agent/agentproc/api_test.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -1110,7 +1111,7 @@ func TestChatIsolation(t *testing.T) {
 	}
 }
 
-func TestHandleStartProcess_ChatHeaders_EmptyWorkDir_StillNotifies(t *testing.T) {
+func TestHandleStartProcess_ChatHeaders_EmptyWorkDir_TracksDefaultDir(t *testing.T) {
 	t.Parallel()
 
 	pathStore := agentgit.NewPathStore()
@@ -1118,10 +1119,11 @@ func TestHandleStartProcess_ChatHeaders_EmptyWorkDir_StillNotifies(t *testing.T)
 	ch, unsub := pathStore.Subscribe(chatID)
 	defer unsub()
 
+	defaultDir := t.TempDir()
 	logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
-	api := agentproc.NewAPI(logger, agentexec.DefaultExecer, nil, pathStore, nil, func(current []string) ([]string, error) {
+	api := agentproc.NewAPI(logger, agentexec.DefaultExecer, afero.NewOsFs(), pathStore, nil, func(current []string) ([]string, error) {
 		return current, nil
-	}, nil)
+	}, func() string { return defaultDir })
 	defer api.Close()
 
 	routes := agentchat.Middleware(api.Routes())
@@ -1138,16 +1140,15 @@ func TestHandleStartProcess_ChatHeaders_EmptyWorkDir_StillNotifies(t *testing.T)
 
 	require.Equal(t, http.StatusOK, rw.Code)
 
-	// The subscriber should be notified even though no paths
-	// were added.
+	// A command without a workdir runs in the default directory,
+	// which counts as touched by the chat.
 	select {
 	case <-ch:
 	case <-time.After(testutil.WaitShort):
 		t.Fatal("timed out waiting for path store notification")
 	}
 
-	// No paths should have been stored for this chat.
-	require.Nil(t, pathStore.GetPaths(chatID))
+	require.Equal(t, []string{defaultDir}, pathStore.GetPaths(chatID))
 }
 
 func TestProcessLifecycle(t *testing.T) {