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_md_load.py

@@ -11,8 +11,7 @@ import textwrap
 import unittest
 from pathlib import Path
 
-from bot_bottle.log import Die
-from bot_bottle.manifest import Manifest
+from bot_bottle.manifest import ManifestError, Manifest
 
 
 def _write(p: Path, text: str) -> None:
@@ -238,7 +237,7 @@ class TestUnknownAgentKeyDies(_ResolveCase):
             ...
             """,
         )
-        with self.assertRaises(Die):
+        with self.assertRaises(ManifestError):
             self.resolve()
 
 
@@ -257,7 +256,7 @@ class TestUnknownBottleKeyDies(_ResolveCase):
             """,
         )
         _write(self.home_cb / "agents" / "implementer.md", _AGENT_IMPL)
-        with self.assertRaises(Die):
+        with self.assertRaises(ManifestError):
             self.resolve()
 
 
@@ -268,7 +267,7 @@ class TestStaleJsonDies(_ResolveCase):
 
     def test_dies(self):
         (self.home_root / "bot-bottle.json").write_text('{"bottles": {}}')
-        with self.assertRaises(Die):
+        with self.assertRaises(ManifestError):
             self.resolve()
 
 
@@ -277,7 +276,7 @@ class TestNoManifestDies(_ResolveCase):
     pointer at the new layout."""
 
     def test_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(ManifestError):
             self.resolve()
 
     def test_missing_ok_returns_empty_manifest(self):
@@ -300,7 +299,7 @@ class TestUnknownBottleReferenceDies(_ResolveCase):
             ---
             """,
         )
-        with self.assertRaises(Die):
+        with self.assertRaises(ManifestError):
             self.resolve()