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

bot_bottle/cli/info.py

@@ -14,8 +14,9 @@ def cmd_info(argv: list[str]) -> int:
     parser.add_argument("name", help="agent name defined in bot-bottle.json")
     args = parser.parse_args(argv)
 
-    manifest = Manifest.resolve(USER_CWD)
-    manifest.require_agent(args.name)
+    names = Manifest.resolve(USER_CWD)
+    names.require_agent(args.name)
+    manifest = names.load_for_agent(args.name)
 
     agent = manifest.agents[args.name]
     bottle = manifest.bottle_for(args.name)

bot_bottle/cli/list.py

@@ -41,7 +41,7 @@ def cmd_list(argv: list[str]) -> int:
 
     if args.scope == "available":
         manifest = Manifest.resolve(USER_CWD)
-        for name in manifest.agents.keys():
+        for name in manifest.all_agent_names:
             print(name)
         return 0