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/completion.go

@@ -0,0 +1,63 @@
+package cmd
+
+import (
+	"path/filepath"
+
+	"github.com/spf13/cobra"
+	"github.com/warrenronsiek/ctask/internal/config"
+	"github.com/warrenronsiek/ctask/internal/workspace"
+)
+
+// completionFilter selects which workspaces a command's tab-completion should
+// surface. It does not affect lookup — that's still the resolver's job — only
+// what the user sees as candidates while typing.
+type completionFilter int
+
+const (
+	completionActive completionFilter = iota
+	completionArchived
+	completionAny
+)
+
+// completeWorkspaces returns a Cobra ValidArgsFunction that enumerates
+// workspace directory basenames matching the requested filter. Basenames are
+// emitted (not bare slugs) because the resolver's exact-match step accepts
+// basenames unambiguously, while bare slugs can collide across categories or
+// dates.
+//
+// On any internal error we surface ShellCompDirectiveError so the shell shows
+// nothing rather than a partial / misleading list.
+func completeWorkspaces(filter completionFilter) func(*cobra.Command, []string, string) ([]string, cobra.ShellCompDirective) {
+	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		// Only the first positional argument is a workspace name.
+		if len(args) > 0 {
+			return nil, cobra.ShellCompDirectiveNoFileComp
+		}
+
+		roots := config.SearchRoots()
+		results, err := workspace.ListWorkspaces(roots, workspace.ListOpts{
+			IncludeArchived: filter != completionActive,
+		})
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+
+		var names []string
+		for _, ws := range results {
+			switch filter {
+			case completionActive:
+				if ws.Meta.Status == "archived" {
+					continue
+				}
+			case completionArchived:
+				if ws.Meta.Status != "archived" {
+					continue
+				}
+			case completionAny:
+				// no filter
+			}
+			names = append(names, filepath.Base(ws.Path))
+		}
+		return names, cobra.ShellCompDirectiveNoFileComp
+	}
+}