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/tool-detail-page.js

@@ -1,10 +1,14 @@
 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 { fetchToolDetail, renderToolBody, STATUS_ICON } from './shared/tool-detail-view.js';
 
 const PAGE_ID = 'tool_detail';
 
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@tool_detail';
+
 function idFromHash() {
   const h = location.hash;
   const prefix = `#${PAGE_ID}?id=`;
@@ -45,20 +49,32 @@ export class ToolDetailPage extends LightElement {
       this._open = e.detail.page === PAGE_ID;
       this.style.display = this._open ? 'flex' : 'none';
       if (this._open) this._loadFromHash();
+      else clearSlice(VIEW_SLICE);
     });
     window.addEventListener('hashchange', () => {
       if (this._open) this._loadFromHash();
     });
   }
 
+  disconnectedCallback() {
+    clearSlice(VIEW_SLICE);
+    super.disconnectedCallback();
+  }
+
   async _loadFromHash() {
     const id = idFromHash();
-    if (id == null) return;
+    if (id == null) { clearSlice(VIEW_SLICE); return; }
     this._loading = true;
     this._error = null;
     this._tool = null;
     try {
       this._tool = await fetchToolDetail(id);
+      // The entity slice: which tool this call ran. Only published on success —
+      // the hash carries the call's id, which tells the model nothing.
+      setSlice(VIEW_SLICE, [{
+        label: 'Open tool call',
+        value: this._tool.display_name || this._tool.name,
+      }]);
     } catch (e) {
       this._error = e.message || String(e);
     } finally {