fix(files): serve remote-case attachments, the path a click takes outside the case

A clicked path that points OUTSIDE the case directory goes through the attachment
routes (the frontend's `_isExternalPreviewPath` sends every absolute path not under
`workingDir` to `POST /attachments`), and those had the same local-`fs` assumption
as file-raw: `realpathSync`/`fs.stat` on a path that only exists on the remote host,
so the file never opened — the case the #415 report was actually about.

- `registerExternalAttachment()` accepts `remote` and resolves through
  `remoteProbePaths` (canonical path, size/mtime, kind, plus the workspace root for
  the confinement check). Everything around it — blocklist, extension allowlist,
  workspace confinement, registry/dedupe — is now shared by both branches, so the
  remote path cannot drift from the local one.
- The by-id routes (`raw`, `preview`, `thumbnail`), the metadata poll and the
  attachment history list resolve over ssh too. `raw` streams with the same
  Range contract as file-raw; `preview` (office) and `thumbnail` answer 400 for a
  remote record; an unreachable host answers 502, a vanished file 404.
- Which host a record is read from follows the SESSION, never the path string: the
  same absolute path is a different file on each host, and a remote session never
  falls back to a local file with that name.
- Codex generated artifacts keep force-workspace confinement for a remote case: the
  well-known artifact directories are anchored at THIS host's home, so only a file
  inside the remote workspace is trusted.

Still local-only by design: writes, office conversion, thumbnails, the file
tree/picker and tail-file.

src/web/server.ts

@@ -1697,10 +1697,12 @@ export class WebServer extends EventEmitter {
             sessionId,
             filePath,
             sessionWorkingDir: session.workingDir,
+            remote: session.remote,
           })
         : await registerExternalAttachment(sessionId, filePath, {
             sessionWorkingDir: session.workingDir,
             forceWorkspaceConfinement: true,
+            remote: session.remote,
           });
     const record = attachmentRegistry.get(sessionId, event.attachmentId);
     if (record) {