polish(v0.5.3): v0.4 lease-prompt hint, invocation-name in user-facing commands, smoke-checklist fixes

Three independent v0.5.3 polish items surfaced during manual WSL smoke testing,
bundled into one commit since they're all string/UX touches with no behavior change.

1. v0.4 lease-prompt tightening (`internal/session/{lease,run,run_preflight}.go`,
   `cmd/entry.go`): when ctask is about to enter direct mode on a workspace that
   already has a live tmux session, the "Continue anyway?" prompt now suggests
   `ctask attach <slug>` as the reattach path. Threaded via
   `PreflightOpts.ActiveLeaseHint` / `LaunchOpts.ActiveLeaseHint`; computed in
   `cmd/entry.go::directModeTmuxHint` (best-effort: silent when no tmux on PATH
   or no session for the workspace). Closes the footgun where a user forgot to
   `export CTASK_SESSION_MODE=persistent` in a second terminal and hit the v0.4
   coexistence prompt without realizing tmux passive-reattach was the intended
   path.

2. Invocation-name in user-facing command hints (`cmd/invocation.go` new,
   `cmd/{persistent,entry,resume,doctor}.go`): the binary name printed in
   bypass / restore / "create one with" suggestions now reflects
   `filepath.Base(os.Args[0])` instead of a hard-coded "ctask". Local-build
   PowerShell users running `.\ctask.exe` see `ctask.exe new <ws> --direct`,
   matching what they need to type. Installed contexts continue to see `ctask`.
   Test seam (`invocationNameOverride`) pins the name to "ctask" in unit tests
   so substring assertions stay stable across Go test binary names. Descriptive
   prose ("ctask persistent mode requires...") and the ssh-remote hint
   (`ssh -t <host> ctask <subcmd>`) intentionally keep the literal "ctask" —
   they refer to the program identity / remote invocation, not the local
   command form. Affected tests: `cmd/{persistent,resume}_test.go` tightened to
   check the full `"<binary> <subcmd> <workspace> --direct"` form.

3. Smoke-checklist fixes (`docs/.../2026-05-08-v0.5.3-smoke-test-checklist.md`):
   six issues caught during the run -- S2 now exports CTASK_SESSION_MODE in
   both terminals (previously only WSL-A had it, which routed WSL-B's
   secondary resume through direct mode instead of passive reattach); O3/A2
   tmux-ls expectations corrected (tmux doesn't emit a literal "(detached)"
   token); P2 expected behavior rewritten (passive reattach detach returns to
   prompt immediately -- AttachExisting calls only shell.AttachSession with
   no PollSessionEnd, the owner is responsible for finalize); A1 no longer
   asks for Ctrl-C in WSL-B; A6 path now uses a glob (was hardcoded to the
   checklist's authoring date, broke on v0.5.1 local-time directory naming);
   M1 PATH-hide rewritten to `$HOME/.local/bin` only (was `:/bin` which is a
   symlink to /usr/bin on usrmerge systems, did not hide tmux) and uses
   `command -v` instead of `which` (which is itself in /usr/bin, unreachable
   under the minimal PATH); M3/M4 reordered so the no-workspace verification
   runs after PATH is restored (was silently failing under minimal PATH);
   T1 wording made "substring match" explicit; T2 wording made N/A
   unambiguous; section 10 split into 10a-10f, each a separate input, to
   work around PSReadLine multi-line paste parsing.

internal/session/lease.go

@@ -159,7 +159,15 @@ func CleanupStaleLease(path string, staleAfter time.Duration) (*Lease, error) {
 
 // FormatActiveWarning renders the human-readable warning printed when a
 // fresh active lease is detected on session start.
-func FormatActiveWarning(l *Lease, now time.Time) string {
+//
+// hint, when non-empty, is rendered between the "may cause conflicts" line
+// and the "Continue anyway?" prompt. Each non-empty line of hint is
+// indented with two spaces to match the rest of the block; callers should
+// supply plain text (no leading indent, no trailing newline required).
+// Used to surface "a tmux session exists; ctask attach <slug> is the
+// reattach path" when ctask is about to enter direct mode on a workspace
+// that already has a persistent tmux session.
+func FormatActiveWarning(l *Lease, now time.Time, hint string) string {
 	startedAgo := now.Sub(l.StartedAt)
 	lastSeenAgo := now.Sub(l.LastHeartbeatAt)
 
@@ -174,6 +182,16 @@ func FormatActiveWarning(l *Lease, now time.Time) string {
 	fmt.Fprintf(&b, "  Last seen: %s ago\n", FormatAgoShort(lastSeenAgo))
 	b.WriteString("\n")
 	b.WriteString("  Opening a second session may cause conflicts.\n")
+	if hint != "" {
+		b.WriteString("\n")
+		for _, line := range strings.Split(strings.TrimRight(hint, "\n"), "\n") {
+			if line == "" {
+				b.WriteString("\n")
+				continue
+			}
+			fmt.Fprintf(&b, "  %s\n", line)
+		}
+	}
 	b.WriteString("  Continue anyway? [y/N] ")
 	return b.String()
 }