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/user-guide/features/plugins.md

@@ -86,7 +86,7 @@ Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable
 | Add CLI commands | `ctx.register_cli_command(name, help, setup_fn, handler_fn)` — adds `hermes <plugin> <subcommand>` |
 | Inject messages | `ctx.inject_message(content, role="user")` — see [Injecting Messages](#injecting-messages) |
 | Ship data files | `Path(__file__).parent / "data" / "file.yaml"` |
-| Bundle skills | Copy `skill.md` to `~/.hermes/skills/` at load time |
+| Bundle skills | `ctx.register_skill(name, path)` — namespaced as `plugin:skill`, loaded via `skill_view("plugin:skill")` |
 | Gate on env vars | `requires_env: [API_KEY]` in plugin.yaml — prompted during `hermes plugins install` |
 | Distribute via pip | `[project.entry-points."hermes_agent.plugins"]` |