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/session-detail.js

@@ -2,9 +2,13 @@ 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';
 
 const PAGE_ID = 'session';
 
+/// The view-context slice this page owns (see `lib/view-context.js`).
+const VIEW_SLICE = 'entity@session';
+
 function formatDate(iso) {
   if (!iso) return '—';
   return new Date(iso).toLocaleString(undefined, {
@@ -64,7 +68,7 @@ export class SessionDetailPage extends LightElement {
       this._open = e.detail.page === PAGE_ID;
       this.style.display = this._open ? 'flex' : 'none';
       if (this._open) this._loadFromHash();
-      else            this._closeWs();
+      else            { this._closeWs(); clearSlice(VIEW_SLICE); }
     };
     this.__onHashChange = () => {
       if (this._open) this._loadFromHash();
@@ -79,6 +83,7 @@ export class SessionDetailPage extends LightElement {
     window.removeEventListener('llm-page-change', this.__onPageChange);
     window.removeEventListener('hashchange',      this.__onHashChange);
     this._closeWs();
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
@@ -90,7 +95,10 @@ export class SessionDetailPage extends LightElement {
 
   _loadFromHash() {
     const id = this._idFromHash();
-    if (id == null) return;
+    if (id == null) { clearSlice(VIEW_SLICE); return; }
+    // The entity slice: which conversation is open. Published before the fetch
+    // — a message sent while the transcript loads is still about this session.
+    setSlice(VIEW_SLICE, [{ label: 'Open conversation', value: `#${id}` }]);
     // Reload on the same id too when the socket is down: leaving the page closes
     // it, so coming back to the session we already hold would otherwise show a
     // frozen snapshot with nothing streaming into it.