refactor: remove core skill direct loading, skills owned by clawhubadapter only

- Drop skill.NewManager from homed bootstrap; skills dir no longer core-managed
- Remove GetInjectedPrompt system-prompt injection (skills are not first-class)
- Delete internal/skill package, SkillAPI, webui /api/v1/skills, status skills block
- ConfigRegistry: plugin config tables now created only via RegisterDef; arbitrary
  scope Set/Get no longer implicitly creates config_<name> tables (fixes stray
  config_today_task table from SKILL directory name being used as a scope)

internal/config/registry_test.go

@@ -121,6 +121,7 @@ func TestPluginConfig(t *testing.T) {
 	r := NewConfigRegistry(path)
 
 	ps := r.PluginConfig("test_deepseek")
+	ps.RegisterDef(ConfigDef{Key: "api_key", Default: "sk-test123"})
 	if err := ps.Set("api_key", "sk-test123"); err != nil {
 		t.Fatalf("PluginSettings.Set: %v", err)
 	}