feat(plugin): marketplace — install plugins from a curated registry

Add a Plugin Manager marketplace that installs external add-ons from a
GitHub repo's Releases. A curated registry (plugins/registry.json in this
repo, extended by PR) lists discoverable plugins; the host fetches it,
shows each entry with a release dropdown, and on install downloads the
asset matching the user's platform plus plugin.toml into the plugins
folder (API-version checked first). Users can also link arbitrary
owner/repo manually. Linked repos persist in settings.

Desktop only (ureq + serde_json); the install path reuses the phase-2
loader. Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/plugin/external.rs

@@ -15,6 +15,14 @@
 
 use std::path::PathBuf;
 
+/// One entry in the curated plugin registry (`plugins/registry.json`).
+#[derive(Debug, Clone)]
+pub struct RegistryEntry {
+    pub repo: String,
+    pub name: String,
+    pub description: String,
+}
+
 /// An add-on package found on disk (not necessarily loaded or compatible).
 #[derive(Debug, Clone)]
 pub struct ExternalPlugin {
@@ -125,7 +133,7 @@ fn lib_present_in(dir: &std::path::Path) -> bool {
 /// host doesn't pull in a full TOML parser for a fixed, host-defined schema.
 /// Returns `None` when the required `id` is missing. `dir` / `lib_present` are
 /// filled in by the caller.
-fn parse_plugin_toml(text: &str) -> Option<ExternalPlugin> {
+pub(crate) fn parse_plugin_toml(text: &str) -> Option<ExternalPlugin> {
     let mut id = None;
     let mut name = String::new();
     let mut version = String::new();