feat(chat): view context — tell the assistant what you're looking at

An eye next to the paperclip shares what the user has open with their next
message: the page, the folder being browsed, the file open in the viewer and
any highlighted passage (line numbers where a source view exists), plus which
entity a detail page is about. The bag is client-authored {label, value} pairs
in English — the backend only clamps (chars, never bytes), neutralizes the
harness tag and renders one <system-extra> block per message, deduped
consecutively so it appears exactly when the view changed. On by default,
per-device toggle, hover/tap to preview, a chip on every sent message;
docs/view-context.md for users, an updated harness.md clause for the model.

web/components/plugin-detail.js

@@ -1,6 +1,7 @@
 import { html, nothing } from 'lit';
 import { LightElement } from '../lib/base.js';
 import { t }            from '../lib/i18n.js';
+import { setSlice, clearSlice } from '../lib/view-context.js';
 import { jf, schemaFields, pluginHealth } from './shared/plugin-common.js';
 
 // One plugin's admin page (`#plugin-detail?id=<plugin id>`), reached from the
@@ -21,6 +22,9 @@ import { jf, schemaFields, pluginHealth } from './shared/plugin-common.js';
 
 const PAGE_ID = 'plugin-detail';
 
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@plugin-detail';
+
 function idFromHash() {
   const m = location.hash.match(/^#plugin-detail\?id=(.*)$/);
   if (!m) return null;
@@ -68,6 +72,7 @@ export class PluginDetailPage extends LightElement {
       this._open = e.detail.page === PAGE_ID;
       this.style.display = this._open ? 'flex' : 'none';
       if (this._open) this._loadFromHash();
+      else clearSlice(VIEW_SLICE);
     });
     window.addEventListener('hashchange', () => {
       if (this._open) this._loadFromHash();
@@ -76,15 +81,19 @@ export class PluginDetailPage extends LightElement {
 
   disconnectedCallback() {
     window.removeEventListener('locale-changed', this.__onLocaleChanged);
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
   async _loadFromHash() {
     const id = idFromHash();
-    if (!id) return;
+    if (!id) { clearSlice(VIEW_SLICE); return; }
     // A different plugin must not inherit the previous one's typed config.
     if (id !== this._id) this._reset();
     this._id = id;
+    // The entity slice: which plugin is open. The id is the whole answer — a
+    // detail page says *which* object, never what its config holds.
+    setSlice(VIEW_SLICE, [{ label: 'Open plugin', value: id }]);
     await this._load();
   }