FN-6071: add plugin registry browsing and install actions

Add a curated plugin registry surface to the dashboard plugin manager.

- expose GET /api/plugins/registry with search, category filtering, and installed-state annotations from the dashboard registry manifest
- add registry fetch/install support to the dashboard plugin manager UI, including search, install/manage actions, and related tests
- document plugin registry entries and registry installability in plugin authoring docs and shared concepts

Files changed:
 CONCEPTS.md                                        |   6 +
 docs/PLUGIN_AUTHORING.md                           |  23 ++
 packages/dashboard/app/api/legacy.ts               |  36 +++
 .../dashboard/app/components/PluginManager.css     | 185 ++++++++++++++-
 .../dashboard/app/components/PluginManager.tsx     | 162 ++++++++++++-
 .../PluginManager.install-browse.test.tsx          |   5 +-
 .../__tests__/PluginManager.registry.test.tsx      | 260 +++++++++++++++++++++
 .../components/__tests__/PluginManager.test.tsx    |   3 +-
 .../__tests__/PluginManager.toggle.test.tsx        |   5 +-
 .../src/__tests__/routes-plugin-registry.test.ts   | 156 +++++++++++++
 packages/dashboard/src/plugin-routes.ts            | 115 +++++++++
 packages/dashboard/src/registry-manifest.json      | 143 ++++++++++++
 12 files changed, 1091 insertions(+), 8 deletions(-)

Fusion-Task-Id: FN-6071

Fusion-Task-Lineage: 72c7bdfd-5832-4323-b675-63d0680167d8

packages/dashboard/app/api/legacy.ts

@@ -20,6 +20,7 @@ import type {
   WorkflowStepResult,
   PluginInstallation,
   PluginSetupCheckResult,
+  PluginState,
   PluginUiSlotDefinition,
   PluginUiContributionDefinition,
   PluginDashboardViewDefinition,
@@ -9073,11 +9074,46 @@ export function resetAgentBudget(agentId: string, projectId?: string): Promise<v
 
 // ── Plugin Management ────────────────────────────────────────────────────────
 
+export interface RegistryPluginEntry {
+  id: string;
+  name: string;
+  description: string;
+  version: string;
+  author: string;
+  category: "runtime" | "integration";
+  npmPackage?: string;
+  path?: string;
+  homepage?: string;
+  tags?: string[];
+  installed: boolean;
+  state?: PluginState;
+  installedVersion?: string;
+  canInstall: boolean;
+}
+
 /** Fetch all installed plugins */
 export async function fetchPlugins(projectId?: string): Promise<PluginInstallation[]> {
   return api<PluginInstallation[]>(withProjectId("/plugins", projectId));
 }
 
+/** Fetch curated registry plugins with installed-state metadata */
+export async function fetchPluginRegistry(
+  query?: string,
+  category?: string,
+  projectId?: string,
+): Promise<RegistryPluginEntry[]> {
+  const params = new URLSearchParams();
+  if (query?.trim()) {
+    params.set("q", query.trim());
+  }
+  if (category?.trim()) {
+    params.set("category", category.trim());
+  }
+  const suffix = params.size > 0 ? `?${params.toString()}` : "";
+  const response = await api<{ plugins: RegistryPluginEntry[] }>(withProjectId(`/plugins/registry${suffix}`, projectId));
+  return response.plugins;
+}
+
 /** Fetch a single plugin by ID */
 export async function fetchPluginDetail(id: string, projectId?: string): Promise<PluginInstallation> {
   return api<PluginInstallation>(withProjectId(`/plugins/${encodeURIComponent(id)}`, projectId));