feat(file-preview): open markdown clicked in the terminal in the rendered viewer

An in-workspace .md path clicked in the terminal went to the log viewer
(tail -f, a "streaming" header over raw text), because md was kept out
of FILE_PREVIEW_EXTENSIONS on purpose; every other surface already
rendered it. Owner decision 2026-10-13: markdown is a document to read,
so md and markdown join FILE_PREVIEW_EXTENSIONS and a click or tap opens
the file preview, rendered by default with the MD pill for source. The
main pane, every grid tile and the split's Pane B share the link
provider, so all of them route the same way. Other in-workspace text
(.log, .txt, .json, code) keeps the log viewer and its live follow.

FILE_PATH_LINK_PATTERN now also links `.markdown` and `.htm`, which
were not clickable at all.

Tests: file-preview-markdown.test.ts now asserts md and markdown ARE
routed to the preview (and plain text is not); terminal-touch-tap pins a
tapped .md, .markdown and .html opening the preview while .log, .txt and
.json open the log viewer, and a tile-registered provider routing the
same way against the tile's own session. Docs: CLAUDE.md, the
architecture invariants and the wiki (Working With Files, Mobile Guide).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/constants.js

@@ -1949,7 +1949,7 @@ function computeRewriteScrollLine(input) {
  * a `/g` regex, so {@link absoluteFilePathPattern} mints a fresh one per call.
  */
 const FILE_PATH_LINK_PATTERN =
-  /(\/(?:home|Users|tmp|var|private|opt|mnt|srv|media|data|workspace)\/[^\s"'<>|;&\n\x00-\x1f]*\.(?:log|txt|json|md|ya?ml|csv|xml|sh|py|tsx|ts|jsx|js|mjs|cjs|css|html|toml|ini|sql|png|jpe?g|gif|webp|avif|bmp|ico|svg|pdf|docx|pptx|xlsx|mp4|webm|mov|mp3|wav))\b/g;
+  /(\/(?:home|Users|tmp|var|private|opt|mnt|srv|media|data|workspace)\/[^\s"'<>|;&\n\x00-\x1f]*\.(?:log|txt|json|markdown|md|ya?ml|csv|xml|sh|py|tsx|ts|jsx|js|mjs|cjs|css|html?|toml|ini|sql|png|jpe?g|gif|webp|avif|bmp|ico|svg|pdf|docx|pptx|xlsx|mp4|webm|mov|mp3|wav))\b/g;
 
 /** A fresh, zero-state instance of {@link FILE_PATH_LINK_PATTERN}. */
 function absoluteFilePathPattern() {
@@ -1961,13 +1961,18 @@ function absoluteFilePathPattern() {
  * points at goes to the tail/log viewer, which is the right home for a growing
  * text file and the wrong one for bytes (tailing a PNG shows binary noise).
  *
+ * Markdown is here too (owner decision 2026-10-13): an agent's `.md` is a
+ * document to read, so the preview renders it (its MD pill gives the source)
+ * instead of the log viewer streaming it as raw text. Other text (.log, .txt,
+ * .json, code) keeps the log viewer, which follows a file still being written.
+ *
  * The media entries mirror VIDEO_ATTACHMENT_EXTENSIONS/AUDIO_ATTACHMENT_EXTENSIONS
  * (src/attachment-registry.ts, the single source) — they diverged once and an
  * in-workspace `.m4a` opened as binary noise in the log viewer while the same
  * file in /tmp played fine. test/media-extension-parity.test.ts pins the sync.
  */
 const FILE_PREVIEW_EXTENSIONS = new Set(
-  ('png jpg jpeg gif webp avif bmp ico svg pdf docx pptx xlsx mp4 webm mov m4v ogv mp3 wav ogg oga m4a aac flac opus').split(' ')
+  ('png jpg jpeg gif webp avif bmp ico svg pdf docx pptx xlsx mp4 webm mov m4v ogv mp3 wav ogg oga m4a aac flac opus md markdown').split(' ')
 );
 
 /** Whether a path's extension is one {@link FILE_PREVIEW_EXTENSIONS} covers. */