feat(plugins): install from a subdirectory within a repo (#42963)

Support installing a plugin that lives in a subdirectory of a larger
repo (docs/tests at root, plugin in a subdir) without forcing a
dedicated single-plugin repo.

Identifier syntax:
  owner/repo/path/to/plugin        (shorthand + subpath)
  <url>.git/path/to/plugin         (.git boundary on GitHub-style URLs)
  <url>#path/to/plugin             (explicit fragment, any scheme)

_resolve_git_url now returns (git_url, subdir); _install_plugin_core
reads the manifest from and moves only the subdir, so root-level docs
and tests no longer leak into ~/.hermes/plugins. _resolve_subdir_within
guards against path traversal, missing dirs, and non-directories.

Both the CLI (hermes plugins install) and the dashboard install endpoint
inherit this for free since they share _install_plugin_core. Dashboard
install hint + placeholder updated to advertise the subdir syntax.

Co-authored-by: Austin Pickett <pickett.austin@gmail.com>

web/src/i18n/en.ts

@@ -362,7 +362,7 @@ export const en: Translations = {
     inactive: "inactive",
     installBtn: "Install",
     installHeading: "Install from GitHub / Git URL",
-    installHint: "Use owner/repo shorthand or a full https:// or git@ clone URL.",
+    installHint: "Use owner/repo shorthand or a full https:// or git@ clone URL. For a plugin in a subdirectory, append the path: owner/repo/path/to/plugin (or <url>#path/to/plugin).",
     memoryProviderLabel: "Memory provider",
     missingEnvWarn: "Set these in Keys before the plugin can run:",
     noDashboardTab: "No dashboard tab",

web/src/pages/PluginsPage.tsx

@@ -240,7 +240,7 @@ export default function PluginsPage() {
               <Input
                 className="font-mono-ui lowercase"
                 id="install-url"
-                placeholder="owner/repo or https://..."
+                placeholder="owner/repo, owner/repo/subdir, or https://..."
                 spellCheck={false}
                 value={installId}
                 onChange={(e) => setInstallId(e.target.value)}