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.

src/frontend/api/ws.rs

@@ -404,10 +404,19 @@ async fn handle_socket(
                 // projection (never stored as text), and the UI renders the
                 // command's `display` instead of the expanded `content`.
                 let attachments = client_msg.attachments.clone();
-                let metadata = (!attachments.is_empty() || command_ref.is_some())
+                // Clamped and tag-neutralized here, at the ingress: the megabyte a
+                // Cmd+A can produce must never reach the column, and every later
+                // reader (projection, REST, echo) works on canonical data.
+                let view_context = core_api::message_meta::sanitize_view_context(
+                    client_msg.view_context.clone(),
+                );
+                let metadata = (!attachments.is_empty()
+                    || command_ref.is_some()
+                    || !view_context.is_empty())
                     .then(|| core_api::message_meta::MessageMetadata {
                         attachments: attachments.clone(),
                         command:     command_ref.clone(),
+                        view_context,
                     });
 
                 // No echo here: the `UserMessage` event is emitted when the message is