feat: milestones 6b-fix through 6e — default-editor, files plugin, workspace host, workspaceItems contribution

- Fix PluginCard openProviders display
- Add default-editor plugin (text/markdown/notes-context)
- Add files plugin with workspaceItems placement
- Add workspaceItems contribution point (Go + API + mock + SDK)
- Add WorkspaceHost component for workspace area
- WorkspaceTree dispatches selection event
- Fix default-editor layout to fill container
- Fix PluginCard unsafe .length access
- Add E2E tests: 34/34 pass
- Add bundle execution check to official-plugins check.sh
- Update docs: PLUGIN_RUNTIME, DEV_PLUGINS, MILESTONE_6B/6C/6D plans

frontend/src/lib/shell/WorkspaceHost.svelte

@@ -0,0 +1,163 @@
+<script>
+  import PluginBundleHost from '../plugin-host/PluginBundleHost.svelte';
+  import * as App from '../../../wailsjs/go/api/App';
+
+  export let currentNodeId = '';
+  export let nodes = [];
+
+  let contributions = {};
+  let plugins = [];
+  let workspaceTools = [];
+
+  $: currentNode = nodes.find(n => n.id === currentNodeId) || null;
+  $: if (currentNodeId) loadTools();
+
+  async function loadTools() {
+    try {
+      const [c, p] = await Promise.all([
+        App.GetContributions().catch(() => ({})),
+        App.GetPlugins().catch(() => []),
+      ]);
+      contributions = c || {};
+      plugins = p || [];
+
+      const enabledIds = new Set(
+        plugins.filter(pl => pl.enabled && (pl.status === 'loaded' || pl.status === 'degraded')).map(pl => pl.manifest?.id)
+      );
+
+      workspaceTools = (contributions.workspaceItems || []).filter(tool => enabledIds.has(tool.pluginId));
+    } catch (e) {
+      console.error('[WorkspaceHost] loadTools error:', e);
+    }
+  }
+</script>
+
+<div class="workspace-host">
+  {#if currentNode}
+    <div class="workspace-header">
+      <span class="workspace-title">{currentNode.title}</span>
+      <span class="workspace-type">{currentNode.type}</span>
+    </div>
+
+    {#if workspaceTools.length > 0}
+      <div class="workspace-tools">
+        {#each workspaceTools as tool (tool.id + tool.pluginId)}
+          <div class="workspace-tool">
+            <div class="tool-header">
+              <span class="tool-title">{tool.title || tool.id}</span>
+              <span class="tool-plugin">{tool.pluginId}</span>
+            </div>
+            <div class="tool-content">
+              <PluginBundleHost
+                pluginId={tool.pluginId}
+                componentId={tool.component}
+                componentProps={{ workspaceNodeId: currentNodeId, workspaceNode: currentNode }}
+              />
+            </div>
+          </div>
+        {/each}
+      </div>
+    {:else}
+      <div class="workspace-empty">
+        <p>No workspace tools available</p>
+        <p class="workspace-hint">Install plugins that contribute workspaceItems to see tools here.</p>
+      </div>
+    {/if}
+  {:else}
+    <div class="workspace-empty">
+      <p>Select a workspace node from the sidebar</p>
+    </div>
+  {/if}
+</div>
+
+<style>
+  .workspace-host {
+    min-width: 0;
+    min-height: 0;
+    display: flex;
+    flex-direction: column;
+    height: 100%;
+    background: #1a1a2e;
+  }
+
+  .workspace-header {
+    display: flex;
+    align-items: center;
+    gap: 0.75rem;
+    padding: 0.75rem 1rem;
+    border-bottom: 1px solid #16213e;
+    flex-shrink: 0;
+  }
+
+  .workspace-title {
+    color: #e0e0f0;
+    font-size: 0.95rem;
+    font-weight: 600;
+  }
+
+  .workspace-type {
+    color: #4ecca3;
+    font-size: 0.75rem;
+    padding: 0.1rem 0.4rem;
+    border-radius: 3px;
+    background: #1a2a3a;
+  }
+
+  .workspace-tools {
+    flex: 1;
+    min-height: 0;
+    overflow-y: auto;
+    padding: 0.5rem;
+  }
+
+  .workspace-tool {
+    border: 1px solid #16213e;
+    border-radius: 6px;
+    margin-bottom: 0.5rem;
+    overflow: hidden;
+  }
+
+  .tool-header {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    padding: 0.4rem 0.75rem;
+    background: #12122a;
+    border-bottom: 1px solid #16213e;
+  }
+
+  .tool-title {
+    color: #e0e0f0;
+    font-size: 0.8rem;
+    font-weight: 600;
+  }
+
+  .tool-plugin {
+    color: #666;
+    font-size: 0.65rem;
+    margin-left: auto;
+  }
+
+  .tool-content {
+    min-height: 300px;
+    max-height: 60vh;
+    overflow: auto;
+  }
+
+  .workspace-empty {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    color: #666;
+    gap: 0.5rem;
+  }
+
+  .workspace-hint {
+    font-size: 0.8rem;
+    color: #555;
+    max-width: 300px;
+    text-align: center;
+  }
+</style>