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/marketplace.js

@@ -2,6 +2,7 @@ import { html, nothing }          from 'lit';
 import { unsafeHTML }             from 'lit/directives/unsafe-html.js';
 import { LightElement }           from '../lib/base.js';
 import { t }                      from '../lib/i18n.js';
+import { setSlice, clearSlice }   from '../lib/view-context.js';
 
 // Connector marketplace — blueprint §14/§15.
 //
@@ -17,6 +18,9 @@ import { t }                      from '../lib/i18n.js';
 
 const ADMIN_ID = 'admin';
 
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@marketplace';
+
 async function jf(url, opts) {
   const res = await fetch(url, opts);
   if (!res.ok) throw new Error(await res.text() || `HTTP ${res.status}`);
@@ -64,15 +68,24 @@ export class MarketplacePage extends LightElement {
     window.addEventListener('llm-page-change', (e) => {
       this._open = e.detail.page === 'marketplace';
       this.style.display = this._open ? 'flex' : 'none';
-      if (this._open) this._load();
+      if (this._open) { this._load(); this._publishViewContext(); }
+      else clearSlice(VIEW_SLICE);
     });
   }
 
   disconnectedCallback() {
     window.removeEventListener('locale-changed', this.__onLocaleChanged);
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
+  // The entity slice: the active search filter, if any. An empty box is no
+  // slice at all, and it is the search term — never a card's fields.
+  _publishViewContext() {
+    const q = this._q.trim();
+    setSlice(VIEW_SLICE, q ? [{ label: 'Search', value: q }] : null);
+  }
+
   get _isAdmin() { return this._me?.role_id === ADMIN_ID; }
 
   async _load() {
@@ -209,7 +222,7 @@ export class MarketplacePage extends LightElement {
         <div class="connector-search">
           <i class="bi bi-search"></i>
           <input class="form-control form-control-sm" placeholder=${t('marketplace.filter.search')}
-            .value=${this._q} @input=${(e) => { this._q = e.target.value; }} />
+            .value=${this._q} @input=${(e) => { this._q = e.target.value; this._publishViewContext(); }} />
         </div>
         ${this._segment(t('marketplace.filter.scope'), this._scope, (v) => { this._scope = v; },
           [[t('marketplace.filter.all'), 'all'], [t('marketplace.filter.global'), 'global'], [t('marketplace.filter.per_user'), 'per_user']])}