fix(agent): default process working directory to agent dir or $HOME (#23224)

Processes started via the agent process API inherited the agent's
own working directory (/tmp/coder.xxx) when no WorkDir was
specified. SSH sessions already use a fallback chain: configured
agent directory > $HOME. This wires the same manifest directory
closure into the process manager so the priority is now:

  explicit req.WorkDir > agent configured dir > $HOME

The resolved directory is recorded on the process struct so
ProcessInfo.WorkDir and pathStore notifications reflect where
the process actually ran.

agent/agentproc/api_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"runtime"
 	"strings"
 	"testing"
@@ -97,18 +98,25 @@ func postSignal(t *testing.T, handler http.Handler, id string, req workspacesdk.
 // execer, returning the handler and API.
 func newTestAPI(t *testing.T) http.Handler {
 	t.Helper()
-	return newTestAPIWithUpdateEnv(t, nil)
+	return newTestAPIWithOptions(t, nil, nil)
 }
 
 // newTestAPIWithUpdateEnv creates a new API with an optional
 // updateEnv hook for testing environment injection.
 func newTestAPIWithUpdateEnv(t *testing.T, updateEnv func([]string) ([]string, error)) http.Handler {
 	t.Helper()
+	return newTestAPIWithOptions(t, updateEnv, nil)
+}
+
+// newTestAPIWithOptions creates a new API with optional
+// updateEnv and workingDir hooks.
+func newTestAPIWithOptions(t *testing.T, updateEnv func([]string) ([]string, error), workingDir func() string) http.Handler {
+	t.Helper()
 
 	logger := slogtest.Make(t, &slogtest.Options{
 		IgnoreErrors: true,
 	}).Leveled(slog.LevelDebug)
-	api := agentproc.NewAPI(logger, agentexec.DefaultExecer, updateEnv, nil)
+	api := agentproc.NewAPI(logger, agentexec.DefaultExecer, updateEnv, nil, workingDir)
 	t.Cleanup(func() {
 		_ = api.Close()
 	})
@@ -253,6 +261,100 @@ func TestStartProcess(t *testing.T) {
 		require.Contains(t, resp.Output, "marker.txt")
 	})
 
+	t.Run("DefaultWorkDirIsHome", func(t *testing.T) {
+		t.Parallel()
+
+		// No working directory closure, so the process
+		// should fall back to $HOME. We verify through
+		// the process list API which reports the resolved
+		// working directory using native OS paths,
+		// avoiding shell path format mismatches on
+		// Windows (Git Bash returns POSIX paths).
+		handler := newTestAPI(t)
+
+		homeDir, err := os.UserHomeDir()
+		require.NoError(t, err)
+
+		id := startAndGetID(t, handler, workspacesdk.StartProcessRequest{
+			Command: "echo ok",
+		})
+
+		resp := waitForExit(t, handler, id)
+		require.NotNil(t, resp.ExitCode)
+		require.Equal(t, 0, *resp.ExitCode)
+
+		w := getList(t, handler)
+		require.Equal(t, http.StatusOK, w.Code)
+		var listResp workspacesdk.ListProcessesResponse
+		require.NoError(t, json.NewDecoder(w.Body).Decode(&listResp))
+		var proc *workspacesdk.ProcessInfo
+		for i := range listResp.Processes {
+			if listResp.Processes[i].ID == id {
+				proc = &listResp.Processes[i]
+				break
+			}
+		}
+		require.NotNil(t, proc, "process not found in list")
+		require.Equal(t, homeDir, proc.WorkDir)
+	})
+
+	t.Run("DefaultWorkDirFromClosure", func(t *testing.T) {
+		t.Parallel()
+
+		// The closure provides a valid directory, so the
+		// process should start there. Use the marker file
+		// pattern to avoid path format mismatches on
+		// Windows.
+		tmpDir := t.TempDir()
+		handler := newTestAPIWithOptions(t, nil, func() string {
+			return tmpDir
+		})
+
+		id := startAndGetID(t, handler, workspacesdk.StartProcessRequest{
+			Command: "touch marker.txt && ls marker.txt",
+		})
+
+		resp := waitForExit(t, handler, id)
+		require.NotNil(t, resp.ExitCode)
+		require.Equal(t, 0, *resp.ExitCode)
+		require.Contains(t, resp.Output, "marker.txt")
+	})
+
+	t.Run("DefaultWorkDirClosureNonExistentFallsBackToHome", func(t *testing.T) {
+		t.Parallel()
+
+		// The closure returns a path that doesn't exist,
+		// so the process should fall back to $HOME.
+		handler := newTestAPIWithOptions(t, nil, func() string {
+			return "/tmp/nonexistent-dir-" + fmt.Sprintf("%d", time.Now().UnixNano())
+		})
+
+		homeDir, err := os.UserHomeDir()
+		require.NoError(t, err)
+
+		id := startAndGetID(t, handler, workspacesdk.StartProcessRequest{
+			Command: "echo ok",
+		})
+
+		resp := waitForExit(t, handler, id)
+		require.NotNil(t, resp.ExitCode)
+		require.Equal(t, 0, *resp.ExitCode)
+
+		w := getList(t, handler)
+		require.Equal(t, http.StatusOK, w.Code)
+		var listResp workspacesdk.ListProcessesResponse
+		require.NoError(t, json.NewDecoder(w.Body).Decode(&listResp))
+		var proc *workspacesdk.ProcessInfo
+		for i := range listResp.Processes {
+			if listResp.Processes[i].ID == id {
+				proc = &listResp.Processes[i]
+				break
+			}
+		}
+		require.NotNil(t, proc, "process not found in list")
+		require.Equal(t, homeDir, proc.WorkDir)
+	})
+
 	t.Run("CustomEnv", func(t *testing.T) {
 		t.Parallel()
 
@@ -781,7 +883,7 @@ func TestHandleStartProcess_ChatHeaders_EmptyWorkDir_StillNotifies(t *testing.T)
 	logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
 	api := agentproc.NewAPI(logger, agentexec.DefaultExecer, func(current []string) ([]string, error) {
 		return current, nil
-	}, pathStore)
+	}, pathStore, nil)
 	defer api.Close()
 
 	routes := api.Routes()