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/models-hub.js

@@ -1,6 +1,10 @@
 import { html } from 'lit';
 import { LightElement } from '../lib/base.js';
 import { t }             from '../lib/i18n.js';
+import { setSlice, clearSlice } from '../lib/view-context.js';
+
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@models';
 
 const CARDS = [
   {
@@ -52,16 +56,28 @@ export class ModelsHubPage extends LightElement {
       this.style.display = open ? 'flex' : 'none';
       if (open) {
         this._section = this._sectionFromHash();
+        this._publishViewContext();
         if (!this._section) this._loadCounts();
+      } else {
+        clearSlice(VIEW_SLICE);
       }
     });
   }
 
   disconnectedCallback() {
     window.removeEventListener('locale-changed', this.__onLocaleChanged);
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
+  // The entity slice: which section is open. The hub root is no one section,
+  // so it publishes nothing — the route slice already describes it.
+  _publishViewContext() {
+    setSlice(VIEW_SLICE, this._section
+      ? [{ label: 'Open section', value: this._section }]
+      : null);
+  }
+
   _sectionFromHash() {
     const parts = location.hash.slice(1).split('/');
     if (parts[0] === 'models' && parts[1]) {
@@ -100,11 +116,13 @@ export class ModelsHubPage extends LightElement {
 
   _openSection(id) {
     this._section = id;
+    this._publishViewContext();
     history.pushState({ page: 'models', section: id }, '', `#models/${id}`);
   }
 
   _goBack() {
     this._section = null;
+    this._publishViewContext();
     this._loadCounts();
     history.replaceState({ page: 'models' }, '', '#models');
   }