refactor: scan filenames at resolve, parse only selected agent at preflight

Manifest.resolve() now returns an empty-dict manifest with only directory
paths recorded (home_md, cwd_md). No content is read from any .md file
until load_for_agent() is called for a specific agent at preflight.

- Manifest.from_md_dirs: scan-only, no frontmatter parsing
- Manifest.load_for_agent: parses the selected agent file and its bottle
  chain; works on eager (from_json_obj) manifests too by returning self
- Manifest.all_agent_names: scans filenames in lazy mode
- backend._validate: calls load_for_agent and propagates upgraded spec
- cli/info.py, cli/list.py, cli/start.py: use load_for_agent / all_agent_names
- manifest_extends.py: reverted to original (no partial-resolve helpers)
- manifest_loader.py: only scan_agent_names + load_bottle_chain_from_dir
- Tests updated to call load_for_agent before accessing agents/bottles;
  test_md_agent_repos_deferred renamed to test_md_agent_repos_fails_at_preflight

tests/unit/test_manifest_agent_git_user.py

@@ -217,7 +217,7 @@ class TestAgentGitUserMdLoader(unittest.TestCase):
     def test_md_agent_git_user_overlays_bottle(self):
         self._write("bottles/dev.md", _BOTTLE_DEV)
         self._write("agents/impl.md", _AGENT_WITH_GIT)
-        m = Manifest.resolve(str(self.home))
+        m = Manifest.resolve(str(self.home)).load_for_agent("impl")
         u = m.bottle_for("impl").git_user
         self.assertEqual("agent-name", u.name)
         self.assertEqual("bottle@example.com", u.email)
@@ -226,16 +226,17 @@ class TestAgentGitUserMdLoader(unittest.TestCase):
             m.git_identity_summary("impl"),
         )
 
-    def test_md_agent_repos_deferred(self):
-        """git-gate.repos on an agent is an error, but deferred into
-        broken_agents rather than raised at resolve time, so other agents
-        remain accessible."""
+    def test_md_agent_repos_fails_at_preflight(self):
+        """git-gate.repos on an agent is an error; resolve() still succeeds
+        so other agents remain accessible, but load_for_agent raises."""
         self._write("bottles/dev.md", _BOTTLE_DEV)
         self._write("agents/impl.md", _AGENT_WITH_REPOS)
-        m = Manifest.resolve(str(self.home))
-        self.assertNotIn("impl", m.agents)
-        self.assertIn("impl", m.broken_agents)
-        msg = str(m.broken_agents["impl"])
+        from bot_bottle.manifest import ManifestError
+        names = Manifest.resolve(str(self.home))
+        self.assertIn("impl", names.all_agent_names)
+        with self.assertRaises(ManifestError) as ctx:
+            names.load_for_agent("impl")
+        msg = str(ctx.exception)
         self.assertIn("git-gate.repos", msg)
         self.assertIn("bottle-only", msg)