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/file-explorer.js

@@ -2,6 +2,14 @@ import { html, nothing } from 'lit';
 import { LightElement } from '../../lib/base.js';
 import { t }            from '../../lib/i18n.js';
 import { fileWatcher }  from '../../lib/file-watcher.js';
+import { setSlice, clearSlice } from '../../lib/view-context.js';
+
+/// The view-context slice this component owns (see `lib/view-context.js`).
+/// One explorer is on screen at a time — `#files` renders `nothing` when it is
+/// not the open page, and the project board is a page of its own — so a single
+/// key is right and two explorers cannot both claim it.
+const VIEW_SLICE = 'path';
+const VIEW_LABEL = 'Open folder';
 
 /// A live file explorer over one subtree of the caller's namespace.
 ///
@@ -29,6 +37,18 @@ import { fileWatcher }  from '../../lib/file-watcher.js';
 /// fires only for a click, never for a `rel` the host itself set — so echoing
 /// the event back as a property is a no-op, and a host that ignores the event
 /// entirely (`project-board.js`) still gets a working explorer.
+///
+/// **It also tells the assistant which folder is open.** The current directory
+/// is published as the `path` view-context slice, in agent-path vocabulary — the
+/// same string the fs-tools take — so "what is in this folder?" needs no
+/// explaining. That it works inside the project board as well as on `#files` is
+/// the whole reason the store is push-based: nobody threads a handle down here.
+/// The slice is cleared on `disconnectedCallback`, which is when both hosts drop
+/// the element (`#files` renders `nothing` while it is not the open page, and so
+/// does the projects page). The board hiding the explorer behind its *Sharing*
+/// tab leaves the slice standing, deliberately: the project's folder is still
+/// the folder the page is about, and which tab is showing is the board's own
+/// slice to publish.
 export class FileExplorer extends LightElement {
   static properties = {
     /// Agent path of the subtree to browse (`~`, `shared/x`, `projects/a/b`,
@@ -80,6 +100,7 @@ export class FileExplorer extends LightElement {
   disconnectedCallback() {
     this._unwatch?.();
     clearTimeout(this._reloadTimer);
+    clearSlice(VIEW_SLICE);
     super.disconnectedCallback();
   }
 
@@ -87,11 +108,20 @@ export class FileExplorer extends LightElement {
     return this._rel ? `${this.root}/${this._rel}` : this.root;
   }
 
+  /// Say which folder is open, before the listing lands: the path is known the
+  /// moment we navigate, and a message sent while the fetch is in flight is
+  /// still a message about *this* folder.
+  _publishViewContext() {
+    if (!this.root) return;
+    setSlice(VIEW_SLICE, [{ label: VIEW_LABEL, value: this._dirPath() }]);
+  }
+
   async _open(rel) {
     this._unwatch?.();
     this._unwatch = null;
     this._rel   = rel;
     this._error = null;
+    this._publishViewContext();
     await this._load();
     // Live updates for the open directory (best-effort: a dead watcher just
     // means manual refresh; auto-reconnect + re-subscribe are handled inside).