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/llm-requests.js

@@ -1,10 +1,18 @@
 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';
 
 const PAGE_ID   = 'llm-requests';
 const PAGE_SIZE = 20;
 
+/// The view-context slice this page owns (see `lib/view-context.js`). It is
+/// published from here and not from `<llm-request-detail>` on purpose: the
+/// detail stays connected (only `display:none`) while the page is hidden, so
+/// the host — which knows both `_open` and `_detailId` — is the one place that
+/// can guarantee the slice never describes a page nobody is looking at.
+const VIEW_SLICE = 'entity@llm-requests';
+
 function formatDate(iso) {
   if (!iso) return '—';
   return new Date(iso).toLocaleString(undefined, {
@@ -75,14 +83,24 @@ export class LlmRequestsPage extends LightElement {
         this._detailId = id;
         if (id == null && this._items.length === 0) this._fetch(1);
       }
+      this._publishViewContext();
     });
   }
 
   disconnectedCallback() {
     window.removeEventListener('locale-changed', this.__onLocaleChanged);
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
+  // The entity slice: which request is open, if any. On the list there is none
+  // — the route slice already describes it.
+  _publishViewContext() {
+    setSlice(VIEW_SLICE, this._open && this._detailId != null
+      ? [{ label: 'Open LLM request', value: `#${this._detailId}` }]
+      : null);
+  }
+
   _idFromHash() {
     const parts = location.hash.replace('#', '').split('/');
     if (parts[0] === PAGE_ID && parts[1]) {
@@ -94,11 +112,13 @@ export class LlmRequestsPage extends LightElement {
 
   _openDetail(id) {
     this._detailId = id;
+    this._publishViewContext();
     history.pushState({}, '', `#${PAGE_ID}/${id}`);
   }
 
   _back() {
     this._detailId = null;
+    this._publishViewContext();
     history.pushState({}, '', `#${PAGE_ID}`);
     if (this._items.length === 0) this._fetch(1);
   }