feat(v0.5.2): add restore, notes, path commands with completion plumbing

Three new direct-lookup commands per v0.5.2-spec.md:

- ctask restore <ws>   un-archive a workspace (metadata-only flip,
                       mirrors archive's lease guard, refuses to
                       restore an already-active workspace)
- ctask notes <ws>     stream a workspace's notes.md to stdout (raw,
                       no framing, [ctask]-prefixed stderr on error)
                       so AI agents can read prior workspace context
                       through standard shell pipelines
- ctask path <ws>      print the absolute filesystem path of a
                       workspace, OS-native separators, one line

All three resolve archived-inclusive: the user typed a name, so we
find the workspace whether or not it's archived. Listing stays
filtered (active-only by default) per the v0.5.2 design rule
"listing is filtered, direct lookup is comprehensive".

Adds shared completion infrastructure (cmd/completion.go) used by
these commands and wired into the existing workspace-accepting
commands in a follow-up commit. Candidates are workspace directory
basenames (e.g. 2026-04-22_promptvolley) rather than bare slugs
because basenames are unique under the resolver's exact-match step
while slugs can collide across categories or dates.

cmd/path_test.go

@@ -0,0 +1,99 @@
+package cmd
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/warrenronsiek/ctask/internal/workspace"
+)
+
+// callPath invokes runPath with stdout captured.
+func callPath(t *testing.T, root, query string) (string, error) {
+	t.Helper()
+
+	prevRoot := os.Getenv("CTASK_ROOT")
+	os.Setenv("CTASK_ROOT", root)
+	defer func() {
+		if prevRoot == "" {
+			os.Unsetenv("CTASK_ROOT")
+		} else {
+			os.Setenv("CTASK_ROOT", prevRoot)
+		}
+	}()
+
+	r, w, _ := os.Pipe()
+	prevStdout := os.Stdout
+	os.Stdout = w
+	defer func() { os.Stdout = prevStdout }()
+
+	err := runPath(pathCmd, []string{query})
+	w.Close()
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+	return buf.String(), err
+}
+
+func TestPathPrintsAbsolutePath(t *testing.T) {
+	root := t.TempDir()
+	dirName := "2026-04-22_path-active"
+	wsDir := filepath.Join(root, "general", dirName)
+	os.MkdirAll(wsDir, 0755)
+	now := time.Now().UTC().Truncate(time.Second)
+	meta := &workspace.TaskMeta{
+		ID: "t", Slug: "path-active", Title: "path-active",
+		CreatedAt: now, UpdatedAt: now,
+		Status: "active", Category: "general", Type: "task",
+		Mode: "local", Agent: "claude",
+	}
+	workspace.WriteMeta(filepath.Join(wsDir, "task.yaml"), meta)
+
+	out, err := callPath(t, root, "path-active")
+	if err != nil {
+		t.Fatalf("path: %v", err)
+	}
+	got := strings.TrimRight(out, "\r\n")
+	want, _ := filepath.Abs(wsDir)
+	if got != want {
+		t.Errorf("output path mismatch:\nwant: %q\ngot:  %q", want, got)
+	}
+	if !strings.HasSuffix(out, "\n") {
+		t.Errorf("output should end with a newline, got: %q", out)
+	}
+	if filepath.IsAbs(got) != true {
+		t.Errorf("output must be absolute, got: %q", got)
+	}
+}
+
+func TestPathWorksOnArchivedWorkspace(t *testing.T) {
+	root := t.TempDir()
+	dirName := "2026-04-22_path-archived"
+	wsDir := filepath.Join(root, "general", dirName)
+	os.MkdirAll(wsDir, 0755)
+	now := time.Now().UTC().Truncate(time.Second)
+	meta := &workspace.TaskMeta{
+		ID: "t", Slug: "path-archived", Title: "path-archived",
+		CreatedAt:  now,
+		UpdatedAt:  now,
+		ArchivedAt: &now,
+		Status:     "archived",
+		Category:   "general",
+		Type:       "task",
+		Mode:       "local",
+		Agent:      "claude",
+	}
+	workspace.WriteMeta(filepath.Join(wsDir, "task.yaml"), meta)
+
+	out, err := callPath(t, root, "path-archived")
+	if err != nil {
+		t.Fatalf("path should find archived workspace: %v", err)
+	}
+	got := strings.TrimRight(out, "\r\n")
+	want, _ := filepath.Abs(wsDir)
+	if got != want {
+		t.Errorf("output path mismatch:\nwant: %q\ngot:  %q", want, got)
+	}
+}