test(manifest): cover lazy (on-disk) loader branches

The eager from_json_obj path is unit-tested; the lazy resolve()/
from_md_dirs path was only hit by the integration suite, so a critical
module relied on Docker for branch coverage. Add tmp-dir tests driving:
all_agent_names with a cwd overlay, load_for_agent on unknown and
malformed-frontmatter agent files, and require_agent's names-only
file-existence checks (home + cwd).

manifest.py: 86% -> 99%. The one remaining line is the OSError branch on
an unreadable agent file (not reliably triggerable cross-environment).

Closes #304

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NkwFXLFff9PYPy4wgVBJp9

tests/unit/test_manifest_lazy_loader.py

@@ -0,0 +1,112 @@
+"""Unit: lazy (on-disk) ManifestIndex loader branches (coverage ratchet).
+
+The eager from_json_obj path is covered by test_manifest_validation.py;
+this drives the lazy resolve()/from_md_dirs path — all_agent_names with a
+cwd overlay, load_for_agent on an unknown / malformed agent file, and
+require_agent's names-only file-existence checks — so manifest.py's
+core-module coverage doesn't depend on the integration suite."""
+
+from __future__ import annotations
+
+import os
+import shutil
+import tempfile
+import textwrap
+import unittest
+from pathlib import Path
+
+from bot_bottle.manifest import ManifestError, ManifestIndex
+
+
+def _write(p: Path, text: str) -> None:
+    p.parent.mkdir(parents=True, exist_ok=True)
+    p.write_text(textwrap.dedent(text).lstrip("\n"))
+
+
+_BOTTLE_DEV = """
+    ---
+    egress:
+      routes:
+        - host: example.com
+    ---
+    The dev bottle.
+"""
+
+_AGENT = """
+    ---
+    bottle: dev
+    ---
+    An agent.
+"""
+
+# Tab in the frontmatter indent -> YamlSubsetError on parse.
+_AGENT_BAD_FM = "---\nskills:\n\t- x\n---\nbody\n"
+
+
+class _LazyCase(unittest.TestCase):
+    def setUp(self) -> None:
+        self.home_root = Path(tempfile.mkdtemp(prefix="cb-home-"))
+        self.cwd_root = Path(tempfile.mkdtemp(prefix="cb-cwd-"))
+        self._orig_home = os.environ.get("HOME")
+        os.environ["HOME"] = str(self.home_root)
+
+    def tearDown(self) -> None:
+        if self._orig_home is None:
+            os.environ.pop("HOME", None)
+        else:
+            os.environ["HOME"] = self._orig_home
+        shutil.rmtree(self.home_root, ignore_errors=True)
+        shutil.rmtree(self.cwd_root, ignore_errors=True)
+
+    @property
+    def home_cb(self) -> Path:
+        return self.home_root / ".bot-bottle"
+
+    @property
+    def cwd_cb(self) -> Path:
+        return self.cwd_root / ".bot-bottle"
+
+    def resolve(self) -> ManifestIndex:
+        return ManifestIndex.resolve(str(self.cwd_root))
+
+
+class TestAllAgentNamesLazy(_LazyCase):
+    def test_merges_home_and_cwd_agents(self) -> None:
+        _write(self.home_cb / "bottles" / "dev.md", _BOTTLE_DEV)
+        _write(self.home_cb / "agents" / "alpha.md", _AGENT)
+        _write(self.cwd_cb / "agents" / "beta.md", _AGENT)
+        self.assertEqual(["alpha", "beta"], self.resolve().all_agent_names)
+
+
+class TestLoadForAgentLazy(_LazyCase):
+    def test_unknown_agent_raises(self) -> None:
+        _write(self.home_cb / "agents" / "alpha.md", _AGENT)
+        with self.assertRaises(ManifestError):
+            self.resolve().load_for_agent("nope")
+
+    def test_malformed_frontmatter_raises(self) -> None:
+        _write(self.home_cb / "bottles" / "dev.md", _BOTTLE_DEV)
+        _write(self.home_cb / "agents" / "broken.md", _AGENT_BAD_FM)
+        with self.assertRaises(ManifestError):
+            self.resolve().load_for_agent("broken")
+
+
+class TestRequireAgentLazy(_LazyCase):
+    def test_existing_home_agent_ok(self) -> None:
+        _write(self.home_cb / "agents" / "alpha.md", _AGENT)
+        self.resolve().require_agent("alpha")  # no raise
+
+    def test_existing_cwd_agent_ok(self) -> None:
+        # File only under cwd -> require_agent's cwd_path branch.
+        _write(self.home_cb / "agents" / "alpha.md", _AGENT)
+        _write(self.cwd_cb / "agents" / "beta.md", _AGENT)
+        self.resolve().require_agent("beta")  # no raise
+
+    def test_unknown_agent_raises(self) -> None:
+        _write(self.home_cb / "agents" / "alpha.md", _AGENT)
+        with self.assertRaises(ManifestError):
+            self.resolve().require_agent("nope")
+
+
+if __name__ == "__main__":
+    unittest.main()