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/cli/commands.py

@@ -591,6 +591,7 @@ def serve(
         channels_config=runtime_config.channels,
         timezone=runtime_config.agents.defaults.timezone,
         unified_session=runtime_config.agents.defaults.unified_session,
+        disabled_skills=runtime_config.agents.defaults.disabled_skills,
     )
 
     model_name = runtime_config.agents.defaults.model
@@ -683,6 +684,7 @@ def gateway(
         channels_config=config.channels,
         timezone=config.agents.defaults.timezone,
         unified_session=config.agents.defaults.unified_session,
+        disabled_skills=config.agents.defaults.disabled_skills,
     )
 
     # Set cron callback (needs agent)
@@ -915,6 +917,7 @@ def agent(
         channels_config=config.channels,
         timezone=config.agents.defaults.timezone,
         unified_session=config.agents.defaults.unified_session,
+        disabled_skills=config.agents.defaults.disabled_skills,
     )
     restart_notice = consume_restart_notice_from_env()
     if restart_notice and should_show_cli_restart_notice(restart_notice, session_id):