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

agent/skill_utils.py

@@ -10,7 +10,7 @@ import os
 import re
 import sys
 from pathlib import Path
-from typing import Any, Dict, List, Set, Tuple
+from typing import Any, Dict, List, Optional, Set, Tuple
 
 from hermes_constants import get_config_path, get_skills_dir
 
@@ -441,3 +441,25 @@ def iter_skill_index_files(skills_dir: Path, filename: str):
             matches.append(Path(root) / filename)
     for path in sorted(matches, key=lambda p: str(p.relative_to(skills_dir))):
         yield path
+
+
+# ── Namespace helpers for plugin-provided skills ───────────────────────────
+
+_NAMESPACE_RE = re.compile(r"^[a-zA-Z0-9_-]+$")
+
+
+def parse_qualified_name(name: str) -> Tuple[Optional[str], str]:
+    """Split ``'namespace:skill-name'`` into ``(namespace, bare_name)``.
+
+    Returns ``(None, name)`` when there is no ``':'``.
+    """
+    if ":" not in name:
+        return None, name
+    return tuple(name.split(":", 1))  # type: ignore[return-value]
+
+
+def is_valid_namespace(candidate: Optional[str]) -> bool:
+    """Check whether *candidate* is a valid namespace (``[a-zA-Z0-9_-]+``)."""
+    if not candidate:
+        return False
+    return bool(_NAMESPACE_RE.match(candidate))