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/users-page.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';
 import { connectorIconUrl }       from './shared/connector-common.js';
 
 // Users admin — the list at `#users`, one user's page at `#users/{id}`.
@@ -29,6 +30,12 @@ function avatarColor(name) {
   return `hsl(${h % 360}, 55%, 52%)`;
 }
 
+// The view-context slice this page owns (see `lib/view-context.js`). Qualified
+// per page, like every `entity` contributor: these pages stay in the DOM while
+// hidden, and a shared key would let a page that hides *after* another one
+// shows wipe the fresh slice (listener order on `llm-page-change`).
+const VIEW_SLICE = 'entity@users';
+
 export class UsersPage extends LightElement {
 
   static get properties() {
@@ -84,6 +91,15 @@ export class UsersPage extends LightElement {
     this._connSaved = false;
     this._plugSaved = false;
     this._trgSaved  = false;
+    clearSlice(VIEW_SLICE);
+  }
+
+  // The entity slice: which person's page is open — the username, and never a
+  // profile field (a detail page says *which* object, not what it contains).
+  _publishViewContext() {
+    const u = this._user;
+    setSlice(VIEW_SLICE,
+      this._view === 'user' && u ? [{ label: 'Open user', value: u.username }] : null);
   }
 
   connectedCallback() {
@@ -94,6 +110,7 @@ export class UsersPage extends LightElement {
       this._open = e.detail.page === 'users';
       this.style.display = this._open ? 'flex' : 'none';
       if (this._open) { this._syncViewFromHash(); this._load(); }
+      else clearSlice(VIEW_SLICE);
     });
     window.addEventListener('hashchange', () => {
       if (this._open) this._syncViewFromHash();
@@ -102,6 +119,7 @@ export class UsersPage extends LightElement {
 
   disconnectedCallback() {
     window.removeEventListener('locale-changed', this.__onLocaleChanged);
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
@@ -117,6 +135,7 @@ export class UsersPage extends LightElement {
       this._users = await uRes.json();
       this._roles = await rRes.json();
       if (this._view === 'user') this._enterDetail();
+      this._publishViewContext();
     } catch (e) {
       this._error = e.message;
     }
@@ -137,6 +156,7 @@ export class UsersPage extends LightElement {
       this._userId = id;
       if (this._users) this._enterDetail();
     }
+    this._publishViewContext();
   }
 
   get _user() { return (this._users ?? []).find(u => u.id === this._userId) ?? null; }