feat(plugins): namespaced skill registration for plugin skill bundles

Add ctx.register_skill() API so plugins can ship SKILL.md files under
a 'plugin:skill' namespace, preventing name collisions with built-in
Hermes skills. skill_view() detects the ':' separator and routes to
the plugin registry while bare names continue through the existing
flat-tree scan unchanged.

Key additions:
- agent/skill_utils: parse_qualified_name(), is_valid_namespace()
- hermes_cli/plugins: PluginContext.register_skill(), PluginManager
  skill registry (find/list/remove)
- tools/skills_tool: qualified name dispatch in skill_view(),
  _serve_plugin_skill() with full guards (disabled, platform,
  injection scan), bundle context banner with sibling listing,
  stale registry self-heal
- Hoisted _INJECTION_PATTERNS to module level (dedup)
- Updated skill_view schema description

Based on PR #9334 by N0nb0at. Lean P1 salvage — omits autogen shim
(P2) for a simpler first merge.

Closes #8422

website/docs/guides/work-with-skills.md

@@ -117,6 +117,24 @@ hermes skills list | grep arxiv
 
 ---
 
+## Plugin-Provided Skills
+
+Plugins can bundle their own skills using namespaced names (`plugin:skill`). This prevents name collisions with built-in skills.
+
+```bash
+# Load a plugin skill by its qualified name
+skill_view("superpowers:writing-plans")
+
+# Built-in skill with the same base name is unaffected
+skill_view("writing-plans")
+```
+
+Plugin skills are **not** listed in the system prompt and don't appear in `skills_list`. They're opt-in — load them explicitly when you know a plugin provides one. When loaded, the agent sees a banner listing sibling skills from the same plugin.
+
+For how to ship skills in your own plugin, see [Build a Hermes Plugin → Bundle skills](/docs/guides/build-a-hermes-plugin#bundle-skills).
+
+---
+
 ## Configuring Skill Settings
 
 Some skills declare configuration they need in their frontmatter: