refactor(manifest): raise ManifestError instead of die()

Review feedback on #102: a manifest that can't be read should raise an
exception, not call die() (a SystemExit). That SystemExit was the whole
reason the dashboard had to special-case Die.

manifest.py now raises ManifestError (a plain Exception) for every
validation failure. The CLI dispatcher catches it and prints+exits 1
(same UX as before); the dashboard catches it with a normal
`except ManifestError` and degrades to a status-line warning. Manifest
tests assert on ManifestError + its message.

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

tests/unit/test_manifest_runtime.py

@@ -7,8 +7,7 @@ silently ignoring."""
 import unittest
 from typing import Any
 
-from bot_bottle.log import Die
-from bot_bottle.manifest import Bottle, Manifest
+from bot_bottle.manifest import ManifestError, Bottle, Manifest
 
 
 def _manifest_with_runtime(value: object) -> dict[str, Any]:
@@ -32,7 +31,7 @@ class TestManifestRuntimeRemoved(unittest.TestCase):
     def test_any_runtime_value_is_rejected(self):
         for value in ("runsc", "runc", "kata-runtime", "", 42, None):
             with self.subTest(value=value):
-                with self.assertRaises(Die):
+                with self.assertRaises(ManifestError):
                     Manifest.from_json_obj(_manifest_with_runtime(value))