feat(files): play video and audio from outside the workspace too

A clip an agent wrote inside the workspace played with a working scrub bar,
while the same file in /tmp was refused as an unsupported type. The workspace
preview classified media with its own inline extension sets and the attachment
allowlist had no media at all, so the two paths disagreed about what a video is.

- VIDEO_ATTACHMENT_EXTENSIONS and AUDIO_ATTACHMENT_EXTENSIONS now live in
  attachment-registry.ts and are imported by file-content's classification, so
  both paths answer the same. mp4/webm/mov/m4v/ogv and
  mp3/wav/ogg/oga/m4a/aac/flac/opus join the attachment allowlist.
- Real MIME types for those extensions. Without one the raw route falls back to
  application/octet-stream, which a <video> refuses to decode: the player
  renders and then does nothing.
- getAttachmentType() gained the video and audio members of
  AttachmentDetectedType. Attachment cards have no per-type CSS and their
  thumbnail falls back to the type label, since the thumbnailer has no media
  branch and answers 204 rather than spawning a converter.
- The preview overlay's by-id branch renders <video>/<audio> with the same
  markup as the workspace branch, playsinline included. Serving was already
  range-aware, so seeking works.

The image-watcher keeps its own narrow detection list (png/pdf/docx/pptx), so
this does not start popping cards for every video an agent writes. Text types
that are not md or txt (.json, .log, code files) remain out of the allowlist by
choice and still report what is previewable instead.

Verified on an isolated instance: an external mp4 and mp3 both play, seek, and
report the right duration, matching the in-workspace clip exactly, and a click
on an external mp4 in the terminal opens the player with no attachment card.

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

docs/architecture-invariants.md

@@ -130,7 +130,9 @@ A file path an agent prints is a link on both surfaces it can appear on, and cli
 
 **The chat linkifier walks text nodes.** `_linkifyFilePaths()` builds anchors with `createElement`/`textContent` on the rendered subtree, never by rebuilding sanitized markup as a string — the source is model output. Subtrees already inside an `<a>` are skipped (marked autolinks URLs; a nested anchor would swallow the click), and the anchor's text is the path verbatim so "copy code" still yields what the agent printed. `test/response-viewer-file-links.test.ts` pins both properties.
 
-**Out-of-workspace paths go through the attachment routes, not the file routes.** `file-content`/`file-raw` resolve against `workingDir` and 404 anything that escapes it, which is correct and unchanged — but the paths agents most often print (a `/tmp` capture, Claude's own scratchpad, another checkout) are exactly that, so clicking one used to report "File not found" for a file sitting on disk. `openFilePreview()` now detects the case (`_isExternalPreviewPath`, a string compare for ROUTING only; the real decision stays server-side) and registers the path via `POST /api/sessions/:id/attachments` first, rendering by id. ⚠️ That registration passes `notify: false`, which suppresses ONLY the `attachment:detected` broadcast — the guard, the registry entry and the by-id routes are identical either way. Without it every click also popped an attachment card announcing the file already filling the screen. ⚠️ The click is an explicit user action on the **explicit, Origin-guarded** registration route, which is why it may cross the workspace boundary at all; the passive magic-link scanner stays force-confined, and nothing here widens `SUPPORTED_ATTACHMENT_EXTENSIONS` (an out-of-workspace `.svg`/`.bmp` is refused with a message naming what IS previewable).
+**Out-of-workspace paths go through the attachment routes, not the file routes.** `file-content`/`file-raw` resolve against `workingDir` and 404 anything that escapes it, which is correct and unchanged — but the paths agents most often print (a `/tmp` capture, Claude's own scratchpad, another checkout) are exactly that, so clicking one used to report "File not found" for a file sitting on disk. `openFilePreview()` now detects the case (`_isExternalPreviewPath`, a string compare for ROUTING only; the real decision stays server-side) and registers the path via `POST /api/sessions/:id/attachments` first, rendering by id. ⚠️ That registration passes `notify: false`, which suppresses ONLY the `attachment:detected` broadcast — the guard, the registry entry and the by-id routes are identical either way. Without it every click also popped an attachment card announcing the file already filling the screen. ⚠️ The click is an explicit user action on the **explicit, Origin-guarded** registration route, which is why it may cross the workspace boundary at all; the passive magic-link scanner stays force-confined. A type outside `SUPPORTED_ATTACHMENT_EXTENSIONS` (`.svg`, `.bmp`, `.json`, `.log`, code files) is refused with a message naming what IS previewable, rather than the registry's own policy term.
+
+**Media is single-sourced across the two preview paths.** `VIDEO_ATTACHMENT_EXTENSIONS` / `AUDIO_ATTACHMENT_EXTENSIONS` live in `attachment-registry.ts` and are imported by `file-content`'s media classification, so a clip plays identically whether it is in the workspace or reached by id from outside it. They diverged first: the workspace path had its own inline sets and the registry allowlist had no media at all, so a video an agent wrote to `/tmp` was refused as an unsupported type while the same file inside the repo played. ⚠️ Three things have to line up for a player rather than a dead frame: the extension in the allowlist, a **real MIME entry** in `MIME_TYPES` (a `<video>` refuses to decode `application/octet-stream`, which presents as a player that renders and then does nothing), and the range-aware body (`serveRawFile` → `sendFileBody`) that makes the scrub bar work. `getAttachmentType()` returns the `video`/`audio` members of `AttachmentDetectedType` for them; the attachment card has no per-type CSS and its thumbnail falls back to the type label, since `generateFirstPageThumbnail` has no media branch and answers 204. ⚠️ The image-watcher keeps its OWN narrow detection list (`png/pdf/docx/pptx`), so this does not start popping cards for every video an agent writes.
 
 ⚠️ **The preview overlay must outrank the panel that launched it.** `.file-preview-overlay` sits at `z-index: 5100`, above the response viewer (5000) and its backdrop (4999); at its historical 2000 a path clicked in the chat opened the overlay *behind* the chat, which reads as a dead link. It stays below the toast/picker band (10000+) so a "Saved" toast still lands on top.