fix(plugins): widen _sanitize_plugin_name for category-namespaced names

Follow-up to PR #28832 — the dashboard plugin routes now accept slashed
names like `observability/langfuse` and `image_gen/openai`, but
`_sanitize_plugin_name` still rejected forward slash and so dashboard
update + remove on those plugins fell through to '404 not found' even
though they exist on disk.

Adds an opt-in `allow_subdir=True` flag that:
- Permits internal forward slashes (category-namespaced plugin keys
  emitted by `_discover_all_plugins`).
- Strips leading and trailing slashes.
- Still rejects `..` and backslash, and still asserts the resolved
  target lives inside `plugins_dir`.

Opted in at the two read-paths that operate on installed plugins:
`_require_installed_plugin` (CLI update/remove) and
`_user_installed_plugin_dir` (dashboard update/remove). The install
path keeps the default (`allow_subdir=False`) because freshly-cloned
plugins always land top-level under `~/.hermes/plugins/<name>/`.

Adds 6 targeted unit tests covering the new flag's allow/reject matrix.

hermes_cli/plugins_cmd.py

@@ -76,22 +76,42 @@ def _plugins_dir() -> Path:
     return plugins
 
 
-def _sanitize_plugin_name(name: str, plugins_dir: Path) -> Path:
+def _sanitize_plugin_name(
+    name: str,
+    plugins_dir: Path,
+    *,
+    allow_subdir: bool = False,
+) -> Path:
     """Validate a plugin name and return the safe target path inside *plugins_dir*.
 
     Raises ``ValueError`` if the name contains path-traversal sequences or would
     resolve outside the plugins directory.
+
+    ``allow_subdir=True`` permits a single forward slash inside *name* so
+    category-namespaced plugin keys like ``observability/langfuse`` or
+    ``image_gen/openai`` (the registry keys emitted by ``_discover_all_plugins``)
+    can be looked up. ``..`` and backslash are still rejected, leading and
+    trailing slashes are stripped, and the resolved target must still live
+    inside *plugins_dir*. Install paths leave this at the default ``False``
+    because a freshly-cloned plugin always lands top-level under
+    ``~/.hermes/plugins/<name>/``.
     """
     if not name:
         raise ValueError("Plugin name must not be empty.")
 
+    if allow_subdir:
+        name = name.strip("/")
+        if not name:
+            raise ValueError("Plugin name must not be empty.")
+
     if name in {".", ".."}:
         raise ValueError(
             f"Invalid plugin name '{name}': must not reference the plugins directory itself."
         )
 
     # Reject obvious traversal characters
-    for bad in ("/", "\\", ".."):
+    bad_chars = ("\\", "..") if allow_subdir else ("/", "\\", "..")
+    for bad in bad_chars:
         if bad in name:
             raise ValueError(f"Invalid plugin name '{name}': must not contain '{bad}'.")
 
@@ -326,7 +346,7 @@ def _display_removed(name: str, plugins_dir: Path) -> None:
 
 def _require_installed_plugin(name: str, plugins_dir: Path, console) -> Path:
     """Return the plugin path if it exists, or exit with an error listing installed plugins."""
-    target = _sanitize_plugin_name(name, plugins_dir)
+    target = _sanitize_plugin_name(name, plugins_dir, allow_subdir=True)
     if not target.exists():
         installed = ", ".join(d.name for d in plugins_dir.iterdir() if d.is_dir()) or "(none)"
         console.print(
@@ -1508,7 +1528,7 @@ def _user_installed_plugin_dir(name: str) -> Optional[Path]:
     """Resolved path under ``~/.hermes/plugins/<name>`` if it exists."""
     plugins_dir = _plugins_dir()
     try:
-        target = _sanitize_plugin_name(name, plugins_dir)
+        target = _sanitize_plugin_name(name, plugins_dir, allow_subdir=True)
     except ValueError:
         return None
     return target if target.is_dir() else None