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/css/copilot-messages.css

@@ -780,6 +780,144 @@
 
 .attach-chip-remove:hover { background: var(--sidebar-hover); color: #dc2626; }
 
+/* ── View context: composer eye + sent-bubble chip ───────────────────────────
+   The pill belongs to the composer and would sit more naturally in
+   copilot-input.css — but that file is desktop-only, and the eye is the same
+   control on both shells. This stylesheet is the one both index.html and
+   mobile.html load, so both halves of the feature live here together. */
+
+.view-ctx-wrap {
+  position: relative;
+  display: inline-flex;
+}
+
+.view-ctx-btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.25rem;
+  height: 2rem;
+  padding: 0 0.45rem;
+  border: 1px solid transparent;
+  border-radius: 999px;
+  background: transparent;
+  color: var(--placeholder-color);
+  font-size: 0.85rem;
+  line-height: 1;
+  cursor: pointer;
+  transition: background 0.12s, border-color 0.12s, color 0.12s;
+}
+
+.view-ctx-btn:hover {
+  background: var(--sidebar-hover);
+  color: var(--text-primary, #1e293b);
+}
+
+.view-ctx-btn--on {
+  color: var(--accent);
+}
+
+.view-ctx-btn--on:hover {
+  background: rgba(var(--accent-rgb), 0.08);
+  border-color: rgba(var(--accent-rgb), 0.3);
+  color: var(--accent);
+}
+
+.view-ctx-count {
+  font-size: 0.7rem;
+  font-weight: 600;
+  font-variant-numeric: tabular-nums;
+}
+
+.view-ctx-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 99;
+}
+
+.view-ctx-panel {
+  position: absolute;
+  bottom: calc(100% + 6px);
+  left: 0;
+  z-index: 100;
+  width: max-content;
+  max-width: min(28rem, 80vw);
+  max-height: 18rem;
+  overflow-y: auto;
+  padding: 0.5rem 0.65rem;
+  border: 1px solid var(--toolbar-border);
+  border-radius: 0.5rem;
+  background: var(--msg-assistant-bg);
+  box-shadow: 0 4px 14px rgba(0, 0, 0, 0.12);
+  font-size: 0.75rem;
+  /* The composer bubble uses pre-wrap; the panel formats its own values. */
+  white-space: normal;
+}
+
+.view-ctx-panel-title {
+  font-weight: 600;
+  color: var(--text-primary, #1e293b);
+  margin-bottom: 0.35rem;
+}
+
+.view-ctx-empty {
+  color: var(--placeholder-color);
+  line-height: 1.35;
+}
+
+.view-ctx-items {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+}
+
+.view-ctx-item {
+  min-width: 0;
+}
+
+.view-ctx-label {
+  color: var(--placeholder-color);
+  font-size: 0.7rem;
+  text-transform: uppercase;
+  letter-spacing: 0.02em;
+}
+
+/* A selection arrives as it was selected: keep its line breaks, but never let a
+   long unbroken path or word push the panel (or the bubble) wider. */
+.view-ctx-value {
+  white-space: pre-wrap;
+  overflow-wrap: anywhere;
+  line-height: 1.35;
+}
+
+/* The chip inside a sent user bubble: collapsed by default, same muted weight as
+   the reasoning block — it is evidence, not content. */
+.view-ctx-chip {
+  margin-top: 0.4rem;
+  font-size: 0.75rem;
+  white-space: normal;
+}
+
+.view-ctx-chip > summary {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3rem;
+  padding: 0.2rem 0.45rem;
+  border: 1px solid var(--toolbar-border);
+  border-radius: 0.5rem;
+  color: var(--placeholder-color);
+  cursor: pointer;
+  list-style: none;
+  line-height: 1.2;
+}
+
+.view-ctx-chip > summary::-webkit-details-marker { display: none; }
+.view-ctx-chip > summary:hover { border-color: var(--accent); color: var(--accent); }
+
+.view-ctx-chip .view-ctx-items {
+  margin-top: 0.4rem;
+  padding-left: 0.15rem;
+}
+
 /* ── Tool card "view details" (eye) ────────────────────────────────────────── */
 
 .copilot-tool-eye {