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/shared/chat-page.js

@@ -1,7 +1,7 @@
 import { html, nothing } from 'lit';
 import { ChatSession }   from '../../lib/chat-session.js';
 import { t }             from '../../lib/i18n.js';
-import { renderMsg, renderAttachmentChips } from '../copilot-render.js';
+import { renderMsg, renderAttachmentChips, renderViewContextPill } from '../copilot-render.js';
 import { renderTaskStrip } from './agent-tasks.js';
 
 export class ChatPage extends ChatSession {
@@ -223,6 +223,7 @@ export class ChatPage extends ChatSession {
                   title=${t('chat.attach')}
                   @click=${() => this.querySelector('.chat-page-file-input')?.click()}
                 ><i class="bi bi-paperclip"></i></button>
+                ${renderViewContextPill(this)}
                 ${this._providers.length > 1 ? html`
                   <select
                     class="chat-page-model-pill"