fix(dashboard): surface launch/crash failures (#100)

The dashboard runs under curses.wrapper and cmd_dashboard only caught
KeyboardInterrupt, so failures vanished:
- die() prints to stderr, but under curses that lands on the alternate
  screen and is wiped on exit, so config errors gave no reason.
- Die is a SystemExit, so the new-agent flow's `except Exception` never
  caught config errors; they crashed the TUI.
- the startup manifest probe was unguarded.

Now: Die carries its message (+ log.error()); cmd_dashboard re-surfaces
a Die's reason once the terminal is restored and writes any other
crash's traceback to ~/.bot-bottle/logs/dashboard-crash.log; the startup
probe and the new-agent flow degrade a bad config to a status-line
warning instead of crashing.

Closes #100

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/log.py

@@ -14,11 +14,23 @@ def warn(msg: str) -> None:
     print(f"bot-bottle: warning: {msg}", file=sys.stderr)
 
 
+def error(msg: str) -> None:
+    print(f"bot-bottle: error: {msg}", file=sys.stderr)
+
+
 class Die(SystemExit):
     """Raised by die() so callers (and tests) can distinguish a deliberate
-    fatal exit from an unrelated SystemExit."""
+    fatal exit from an unrelated SystemExit.
+
+    Carries the human-facing message so a caller that suppressed stderr
+    — e.g. the curses dashboard, whose alternate screen is wiped when the
+    terminal is restored — can re-surface the reason after the fact."""
+
+    def __init__(self, code: int = 1, message: str = "") -> None:
+        super().__init__(code)
+        self.message = message
 
 
 def die(msg: str) -> NoReturn:
-    print(f"bot-bottle: error: {msg}", file=sys.stderr)
-    raise Die(1)
+    error(msg)
+    raise Die(1, msg)