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.
8cf977c8b1