fix(files): open file paths agents print, from the terminal and the chat

A path an agent prints was already underlined in the terminal, but clicking
one opened the preview overlay on "File not found": file-content/file-raw
resolve against the session workingDir and refuse anything outside it, and the
paths agents print most (a /tmp capture, Claude's own scratchpad, another
checkout) are outside it by definition. In the response viewer those paths were
not links at all.

- openFilePreview() detects an out-of-workspace path and registers it through
  POST /api/sessions/:id/attachments first, rendering by attachment id. That is
  the surface built for live external files, so the server-side guard is
  unchanged: secret trees blocked, symlinks resolved, extension allowlist. The
  workspace routes keep refusing escapes exactly as before.
- New optional `notify` field on that route. `notify: false` suppresses only the
  attachment:detected broadcast, so a click does not also pop a card announcing
  the file already filling the screen. Default stays true for the CLI and
  publish callers.
- _linkifyFilePaths() links paths in rendered response-viewer markdown. It walks
  text nodes and builds anchors with DOM APIs (the source is model output; never
  a string rebuild of sanitized markup), skips subtrees already inside an <a>,
  and keeps the message text byte-identical so copy-code is unaffected.
- One path pattern in constants.js now feeds both the xterm link provider and
  the chat linkifier, a fresh instance per call since lastIndex is per-object
  state. It picks up /Users and /mnt roots (nothing was clickable on macOS or
  WSL), plus docx/pptx and video/audio extensions.
- .file-preview-overlay moves to z-index 5100, above the response viewer at
  5000. At its old 2000 a path clicked in the chat opened the overlay behind the
  panel it was launched from.

Verified end to end on an isolated instance, desktop and phone viewport: real
clicks in the terminal and the chat both render the image, external md and pdf
render, /etc/hosts is still refused, workspace previews unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/styles.css

@@ -9855,13 +9855,18 @@ kbd {
 
 /* ========== File Preview Overlay ========== */
 
+/* Above the response viewer (5000) and its backdrop (4999): a file path in the
+   chat opens this overlay, and at the old 2000 it rendered BEHIND the panel it
+   was launched from — the click looked dead. Same relationship the path picker
+   and its preview already have (10020 / 10030). Still below the toast and
+   picker band (10000+), so a "Saved" toast keeps landing on top. */
 .file-preview-overlay {
   position: fixed;
   inset: 0;
   background: var(--modal-backdrop);
   backdrop-filter: blur(6px);
   -webkit-backdrop-filter: blur(6px);
-  z-index: 2000;
+  z-index: 5100;
   display: none;
   align-items: center;
   justify-content: center;
@@ -12413,6 +12418,16 @@ kbd {
   border-bottom-color: var(--accent);
 }
 
+/* File paths linkified out of the message text. Monospace so a path still reads
+   as a path in prose, and break-all because these are long and the viewer is
+   narrow on a phone. Colour/underline come from the .rv-text a rule above. */
+.rv-text a.rv-path {
+  font-family: 'Fira Code', 'JetBrains Mono', 'SF Mono', Menlo, Monaco, monospace;
+  font-size: 0.92em;
+  word-break: break-all;
+  cursor: pointer;
+}
+
 /* Tables — scroll wrapper keeps table proper while allowing horizontal overflow */
 .rv-table-wrap {
   margin: 1em 0;