fix(skills): apply global|platform disabled union to all resolution sites

The platform-disabled fix landed only in agent.skill_utils.get_disabled_skill_names
(the system-prompt path). Two sibling resolvers still used the old
replace-not-union semantics, so the same skill could be hidden from the
<available_skills> prompt yet reported enabled elsewhere:

- hermes_cli/skills_config.get_disabled_skills (the 'hermes skills config' UI)
  returned only the platform list, so a globally-disabled skill showed as
  enabled (unchecked) on any platform with a platform_disabled entry.
- tools/skills_tool._is_skill_disabled (gates whether skill_view loads a skill)
  ignored the global list when a platform list existed, so a globally-disabled
  skill could still be loaded on such a platform.

Both now union the global list with the platform list, matching
get_disabled_skill_names. An explicit empty platform list no longer re-enables
a globally-disabled skill — global disables hold on every platform (#46201).

Also: fix the now-stale get_disabled_skill_names docstring and drop a stray
blank line. Regression tests added for both sites (proven to fail on the old
replace semantics).

agent/skill_utils.py

@@ -278,8 +278,10 @@ def get_disabled_skill_names(platform: str | None = None) -> Set[str]:
     Args:
         platform: Explicit platform name (e.g. ``"telegram"``).  When
             *None*, resolves from ``HERMES_PLATFORM`` or
-            ``HERMES_SESSION_PLATFORM`` env vars.  Falls back to the
-            global disabled list when no platform is determined.
+            ``HERMES_SESSION_PLATFORM`` env vars.  Returns the global
+            disabled list, unioned with the platform-specific list when a
+            platform is resolved (a globally-disabled skill stays disabled
+            on every platform).
 
     Reads the config file directly (no CLI config imports) to stay
     lightweight.