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/tasks/index.js

@@ -1,5 +1,6 @@
 import { html, nothing } from 'lit';
 import { LightElement } from '../../lib/base.js';
+import { setSlice, clearSlice } from '../../lib/view-context.js';
 import { RunningTasksSection }   from './running.js';
 import { CronJobsSection }       from './cron.js';
 import { ScheduledTasksSection } from './scheduled.js';
@@ -7,6 +8,9 @@ import { TaskHistorySection }    from './history.js';
 
 const SECTIONS = ['running', 'cron', 'scheduled', 'history'];
 
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@tasks';
+
 export class TasksPage extends LightElement {
   static properties = {
     _open:    { state: true },
@@ -29,9 +33,12 @@ export class TasksPage extends LightElement {
         const sec = this._sectionFromHash();
         this._section = sec;
         this._loadSection(sec);
+        this._publishViewContext();
         if (!location.hash.includes('/')) {
           history.replaceState({ page: 'tasks', section: sec }, '', '#tasks/' + sec);
         }
+      } else {
+        clearSlice(VIEW_SLICE);
       }
     });
     window.addEventListener('tasks-section-change', (e) => {
@@ -39,9 +46,20 @@ export class TasksPage extends LightElement {
       const sec = e.detail.section;
       this._section = sec;
       this._loadSection(sec);
+      this._publishViewContext();
     });
   }
 
+  disconnectedCallback() {
+    clearSlice(VIEW_SLICE);
+    super.disconnectedCallback();
+  }
+
+  // The entity slice: which of the four sections is showing.
+  _publishViewContext() {
+    setSlice(VIEW_SLICE, [{ label: 'Open section', value: this._section }]);
+  }
+
   _sectionFromHash() {
     const parts = location.hash.slice(1).split('/');
     if (parts[0] === 'tasks' && parts[1]) {