feat(interface): show resume hint on user-initiated exit

When the user shuts down a run (Ctrl+C in CLI, Ctrl+Q / quit dialog
in TUI, or an uncaught exception during the scan), print a Rich
panel telling them the exact command to pick up where they left off:

    strix --run-name <run_name>

The panel only appears when ``strix_runs/<run_name>/bus.json``
exists — i.e. the scan registered at least the root agent and has
snapshot state worth resuming from. Suppressed when:

  * No run-name was assigned (Ctrl+C before sandbox bring-up).
  * The run dir doesn't exist or has no bus.json yet.

Implementation:

  * ``strix/interface/utils.py`` gains ``format_resume_hint(run_name)
    -> Panel | None``.
  * ``cli.py`` calls it in the SIGINT/SIGTERM/SIGHUP handler before
    ``sys.exit(1)``, and in the ``except Exception`` arm before the
    re-raise.
  * ``tui.py:run_tui`` calls it in a ``finally`` after
    ``app.run_async()`` so the hint lands on the real terminal once
    Textual has restored it (whether the user pressed Ctrl+Q,
    confirmed the quit dialog, or the run completed naturally).

strix/interface/utils.py

@@ -1449,3 +1449,37 @@ def validate_config_file(config_path: str) -> Path:
         sys.exit(1)
 
     return path
+
+
+def format_resume_hint(run_name: str | None) -> Panel | None:
+    """Tell the user how to resume the current run, if it persisted state.
+
+    Returns a Rich :class:`Panel` ready to ``console.print`` when
+    ``strix_runs/<run_name>/bus.json`` exists (i.e. the scan registered
+    at least the root agent and has snapshot state to resume from).
+    Returns ``None`` when there's nothing to resume — e.g. the user hit
+    Ctrl+C before sandbox bring-up, or no run-name was assigned yet.
+
+    Mirrors the auto-resume contract in ``orchestration/scan.py``:
+    re-running with the same ``--run-name`` picks up where this scan
+    left off; deleting ``strix_runs/<run_name>/`` forces a fresh start.
+    """
+    if not run_name:
+        return None
+    run_dir = Path("strix_runs") / run_name
+    if not (run_dir / "bus.json").exists():
+        return None
+
+    text = Text()
+    text.append("Run paused. Resume any time with:\n\n", style="white")
+    text.append(f"  strix --run-name {run_name}\n", style="bold #60a5fa")
+    text.append("\nOr delete ", style="dim")
+    text.append(f"strix_runs/{run_name}/", style="bold dim")
+    text.append(" to start fresh.", style="dim")
+    return Panel(
+        text,
+        title="[bold white]STRIX",
+        title_align="left",
+        border_style="#60a5fa",
+        padding=(1, 2),
+    )