feat(skills): add disabled_skills config to exclude skills from loading

Introduce a disabled_skills option in the config schema that allows
users to specify a list of skill names to be excluded. The setting is
threaded from config through Nanobot -> AgentLoop -> ContextBuilder ->
SkillsLoader. Disabled skills are filtered out from list_skills,
get_always_skills, and build_skills_summary. Four new test cases cover
the filtering behavior.

nanobot/nanobot.py

@@ -82,6 +82,7 @@ class Nanobot:
             mcp_servers=config.tools.mcp_servers,
             timezone=defaults.timezone,
             unified_session=defaults.unified_session,
+            disabled_skills=defaults.disabled_skills,
         )
         return cls(loop)