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.go

@@ -0,0 +1,31 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/warrenronsiek/ctask/internal/config"
+)
+
+var pathCmd = &cobra.Command{
+	Use:          "path <query>",
+	Short:        "Print the absolute filesystem path of a workspace",
+	Args:         cobra.ExactArgs(1),
+	SilenceUsage: true,
+	RunE:         runPath,
+}
+
+func init() {
+	pathCmd.ValidArgsFunction = completeWorkspaces(completionAny)
+	rootCmd.AddCommand(pathCmd)
+}
+
+// runPath prints QueryResult.Path to stdout with a single trailing newline.
+// Uses native path separators — backslashes on Windows, forward slashes on
+// Linux. Output is meant to be consumed by shell pipelines and agent tooling.
+func runPath(cmd *cobra.Command, args []string) error {
+	roots := config.SearchRoots()
+	ws := resolveOne(roots, args[0], true)
+	fmt.Println(ws.Path)
+	return nil
+}