polish(v0.5.4): invocation-name audit + targeted regression tests

Audit walked every cmd/ and internal/ file that produces user-facing
output. All command-form hints (text the user is meant to type back)
were already routed through invocationName() in v0.5.3 — the audit is
a verification pass, not a code rewrite.

Additions:

- Extract formatResumeRestoreHint and formatDirectModeTmuxHint as
  testable string-only helpers. Production paths are unchanged
  behaviorally; the helpers exist purely so the audit can pin down
  the format strings without simulating tmux or stderr capture.

- Two new tests pinning the invocation name to a non-canonical value
  ("my-bin"). The pre-existing tests already protect these surfaces
  but pin the name to "ctask", so they cannot detect a regression
  that hard-codes "ctask" inside the format string. The new tests
  flush that out for the resume restore hint and the Layer-1
  active-session attach hint.

- Drop the duplicated withInvocationName helper accidentally added in
  the info-session tests; reuse the canonical helper from
  persistent_test.go.

Product-identity references ("ctask persistent mode requires tmux",
the SSH-remote `ssh -t <host> ctask <subcmd>` hint, doctor's "[ctask]"
diagnostic prefix, root-command Use:/Long:) deliberately remain
literal per spec §2.

cmd/resume.go

@@ -58,9 +58,7 @@ func doResume(query string, container, useShell, force bool, agentOverride strin
 	ws := resolveOne(roots, query, true)
 
 	if ws.Meta.Status == "archived" {
-		fmt.Fprintf(os.Stderr,
-			"[ctask] error: workspace %q is archived\n\nTo restore it:\n  %s restore %s\n",
-			query, invocationName(), query)
+		fmt.Fprint(os.Stderr, formatResumeRestoreHint(query))
 		return fmt.Errorf("workspace archived")
 	}
 
@@ -88,3 +86,19 @@ func doResume(query string, container, useShell, force bool, agentOverride strin
 		CommandName: "resume",
 	})
 }
+
+// formatResumeRestoreHint builds the multi-line stderr block printed
+// when `ctask resume <query>` resolves to an archived workspace.
+// Extracted so the v0.5.4 invocation-name audit can verify the
+// command-form line uses invocationName() without depending on the
+// surrounding fmt.Fprintf machinery.
+//
+// The "[ctask]" diagnostic prefix is intentionally a literal product
+// reference (spec §2: product-identity references stay literal). The
+// `restore <query>` line is the command-form portion and uses
+// invocationName().
+func formatResumeRestoreHint(query string) string {
+	return fmt.Sprintf(
+		"[ctask] error: workspace %q is archived\n\nTo restore it:\n  %s restore %s\n",
+		query, invocationName(), query)
+}