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/notes.go

@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+
+	"github.com/spf13/cobra"
+	"github.com/warrenronsiek/ctask/internal/config"
+)
+
+var notesCmd = &cobra.Command{
+	Use:           "notes <query>",
+	Short:         "Print a workspace's notes.md to stdout",
+	Args:          cobra.ExactArgs(1),
+	SilenceUsage:  true,
+	SilenceErrors: true,
+	RunE:          runNotes,
+}
+
+func init() {
+	notesCmd.ValidArgsFunction = completeWorkspaces(completionAny)
+	rootCmd.AddCommand(notesCmd)
+}
+
+// runNotes streams <workspace>/notes.md to stdout. SilenceErrors is enabled
+// so the [ctask]-prefixed message we print to stderr is the only diagnostic
+// the user sees — no Cobra "Error: ..." line on top of it.
+func runNotes(cmd *cobra.Command, args []string) error {
+	roots := config.SearchRoots()
+	ws := resolveOne(roots, args[0], true)
+
+	notesPath := filepath.Join(ws.Path, "notes.md")
+	f, err := os.Open(notesPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			fmt.Fprintf(os.Stderr, "[ctask] no notes.md found in workspace %q\n", args[0])
+			return fmt.Errorf("notes.md missing")
+		}
+		fmt.Fprintf(os.Stderr, "[ctask] error reading notes.md: %v\n", err)
+		return err
+	}
+	defer f.Close()
+
+	if _, err := io.Copy(os.Stdout, f); err != nil {
+		fmt.Fprintf(os.Stderr, "[ctask] error streaming notes.md: %v\n", err)
+		return err
+	}
+	return nil
+}