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/system-agents.js

@@ -1,11 +1,15 @@
 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 { ConfigFormController, maybeT, propKeyId } from './shared/config-form.js';
 
 const PAGE_ID  = 'system-agents';
 const PER_PAGE = 20;
 
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@system-agents';
+
 /** The overview tab: every agent's runs, interleaved. */
 const ALL_TAB = '__all__';
 
@@ -98,19 +102,28 @@ export class SystemAgentsPage extends LightElement {
     window.addEventListener('llm-page-change', (e) => {
       this._open = e.detail.page === PAGE_ID;
       this.style.display = this._open ? 'flex' : 'none';
-      if (this._open) this._loadAll();
+      if (this._open) { this._loadAll(); this._publishViewContext(); }
       // Navigating away stops the polling; the pass keeps running server-side
       // and its row is waiting on the next visit.
-      else this._stopPolling();
+      else { this._stopPolling(); clearSlice(VIEW_SLICE); }
     });
   }
 
   disconnectedCallback() {
     window.removeEventListener('locale-changed', this.__onLocaleChanged);
     this._stopPolling();
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
+  // The entity slice: which agent's tab is open. The "All" tab is no one agent,
+  // so it publishes nothing — the route slice already describes the page.
+  _publishViewContext() {
+    setSlice(VIEW_SLICE, this._tab === ALL_TAB
+      ? null
+      : [{ label: 'Open tab', value: this._tab }]);
+  }
+
   async _loadAll() {
     await this._fetchAgents();
     await this._fetch(this._page);
@@ -158,6 +171,7 @@ export class SystemAgentsPage extends LightElement {
     this._tab    = id;
     this._page   = 1;
     this._runMsg = null;
+    this._publishViewContext();
     this._fetch(1);
   }