feat(kanban): warn users that scratch workspaces are deleted on completion (#30949)

First scratch workspace creation on an install now emits a one-shot
warning log + a 'tip_scratch_workspace' event on the task. Sentinel
file at ~/.hermes/kanban/.scratch_tip_shown silences subsequent
creations across the whole install.

Behavior unchanged — scratch is still ephemeral by design. This just
makes the design visible to new users (reported in user community:
'progress files vanished, no warning anywhere').

Docs (en + ko) updated to spell out 'Deleted when the task completes'
on the scratch bullet and 'Preserved on completion' on worktree/dir.

hermes_cli/kanban_db.py

@@ -3094,6 +3094,93 @@ def _cleanup_worker_tmux(conn: sqlite3.Connection, task_id: str) -> None:
         pass  # best-effort — never block completion
 
 
+# ---------------------------------------------------------------------------
+# First-use tip for scratch workspaces
+# ---------------------------------------------------------------------------
+#
+# Scratch workspaces are intentionally ephemeral — ``_cleanup_workspace``
+# removes them as soon as ``complete_task`` runs.  New users often don't
+# realize that and lose worker output (community report, May 2026).  The
+# behavior is right; the lack of warning is the bug.
+#
+# On the FIRST scratch workspace materialization across the whole install
+# we:
+#   1. Log a warning line on the dispatcher logger.
+#   2. Append a ``tip_scratch_workspace`` event on the task so it's visible
+#      via ``hermes kanban show <id>`` and the dashboard.
+#   3. Touch a sentinel file under ``kanban_home() / '.scratch_tip_shown'``
+#      so we don't repeat the tip — once you know, you know.
+#
+# Scope is per-install, not per-board: a user creating a second board
+# already learned the lesson on board #1.
+
+_SCRATCH_TIP_SENTINEL_NAME = ".scratch_tip_shown"
+
+_SCRATCH_TIP_MESSAGE = (
+    "scratch workspaces are ephemeral — they're deleted when the task "
+    "completes. Use --workspace worktree: (git worktree) or "
+    "--workspace dir:/abs/path (existing dir) to preserve worker output."
+)
+
+
+def _scratch_tip_sentinel_path() -> Path:
+    """Path to the per-install scratch-workspace-tip sentinel file."""
+    return kanban_home() / _SCRATCH_TIP_SENTINEL_NAME
+
+
+def _scratch_tip_shown() -> bool:
+    """True iff the scratch-workspace tip has already been emitted on this
+    install. Best-effort — any error means we re-emit, which is the safer
+    failure mode for a help message."""
+    try:
+        return _scratch_tip_sentinel_path().exists()
+    except OSError:
+        return False
+
+
+def _mark_scratch_tip_shown() -> None:
+    """Touch the sentinel so future scratch workspaces stay silent.
+
+    Best-effort: a failure here just means the tip might appear once more,
+    which is preferable to crashing dispatch over a help message.
+    """
+    try:
+        path = _scratch_tip_sentinel_path()
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.touch(exist_ok=True)
+    except OSError:
+        pass
+
+
+def _maybe_emit_scratch_tip(
+    conn: sqlite3.Connection,
+    task_id: str,
+    workspace_kind: Optional[str],
+) -> None:
+    """Emit the first-use scratch-workspace tip exactly once per install.
+
+    Called from the dispatcher right after a scratch workspace is
+    materialized. No-op for ``worktree`` / ``dir`` workspaces (they're
+    preserved by design) and no-op after the sentinel exists.
+    """
+    if (workspace_kind or "scratch") != "scratch":
+        return
+    if _scratch_tip_shown():
+        return
+    try:
+        _log.warning("kanban: %s (task %s)", _SCRATCH_TIP_MESSAGE, task_id)
+        with write_txn(conn):
+            _append_event(
+                conn, task_id, "tip_scratch_workspace",
+                {"message": _SCRATCH_TIP_MESSAGE},
+            )
+    except Exception:
+        # Best-effort — never block the spawn loop over a help message.
+        pass
+    finally:
+        _mark_scratch_tip_shown()
+
+
 def edit_completed_task_result(
     conn: sqlite3.Connection,
     task_id: str,
@@ -5025,6 +5112,7 @@ def dispatch_once(
             continue
         # Persist the resolved workspace path so the worker can cd there.
         set_workspace_path(conn, claimed.id, str(workspace))
+        _maybe_emit_scratch_tip(conn, claimed.id, claimed.workspace_kind)
         _spawn = spawn_fn if spawn_fn is not None else _default_spawn
         try:
             # Back-compat: older spawn_fn signatures accept only
@@ -5103,6 +5191,7 @@ def dispatch_once(
             continue
         # Persist the resolved workspace path so the worker can cd there.
         set_workspace_path(conn, claimed.id, str(workspace))
+        _maybe_emit_scratch_tip(conn, claimed.id, claimed.workspace_kind)
         # Force-load sdlc-review skill for review agents.  The
         # _default_spawn function already auto-loads kanban-worker, and
         # appends task.skills via --skills.  Setting task.skills here