fix: disabled skills respected across banner, system prompt, slash commands, and skill_view (#1897)

* fix: banner skill count now respects disabled skills and platform filtering

The banner's get_available_skills() was doing a raw rglob scan of
~/.hermes/skills/ without checking:
- Whether skills are disabled (skills.disabled config)
- Whether skills match the current platform (platforms: frontmatter)

This caused the banner to show inflated skill counts (e.g. '100 skills'
when many are disabled) and list macOS-only skills on Linux.

Fix: delegate to _find_all_skills() from tools/skills_tool which already
handles both platform gating and disabled-skill filtering.

* fix: system prompt and slash commands now respect disabled skills

Two more places where disabled skills were still surfaced:

1. build_skills_system_prompt() in prompt_builder.py — disabled skills
   appeared in the <available_skills> system prompt section, causing
   the agent to suggest/load them despite being disabled.

2. scan_skill_commands() in skill_commands.py — disabled skills still
   registered as /skill-name slash commands in CLI help and could be
   invoked.

Both now load _get_disabled_skill_names() and filter accordingly.

* fix: skill_view blocks disabled skills

skill_view() checked platform compatibility but not disabled state,
so the agent could still load and read disabled skills directly.

Now returns a clear error when a disabled skill is requested, telling
the user to enable it via hermes skills or inspect the files manually.

---------

Co-authored-by: Test <test@test.com>

tests/agent/test_skill_commands.py

@@ -85,6 +85,21 @@ class TestScanSkillCommands:
             result = scan_skill_commands()
         assert "/generic-tool" in result
 
+    def test_excludes_disabled_skills(self, tmp_path):
+        """Disabled skills should not register slash commands."""
+        with (
+            patch("tools.skills_tool.SKILLS_DIR", tmp_path),
+            patch(
+                "tools.skills_tool._get_disabled_skill_names",
+                return_value={"disabled-skill"},
+            ),
+        ):
+            _make_skill(tmp_path, "enabled-skill")
+            _make_skill(tmp_path, "disabled-skill")
+            result = scan_skill_commands()
+        assert "/enabled-skill" in result
+        assert "/disabled-skill" not in result
+
 
 class TestBuildPreloadedSkillsPrompt:
     def test_builds_prompt_for_multiple_named_skills(self, tmp_path):