fix(files): read remote-case file previews and downloads over ssh

A remote case's workingDir is an absolute path on the remote host, but the
file read routes resolved it with local `fs`: `validateSessionFilePath`'s
realpathSync fails for a path that does not exist on the Codeman host, so
every preview of an agent-written file answered "File not found" (#415).

Add src/remote-files.ts as the single remote-read layer, built on the same
buildSshConnectionArgs() the launch uses:

- remoteProbePaths(): ONE round trip returning realpath + stat for the
  requested path AND the workspace root, so containment is checked against a
  remotely canonicalized root (a symlinked remotePath is ordinary).
- remoteCreateReadStream(): streams the body (cat, or tail -c +N | head -c L
  for a Range) with nothing buffered in memory, and reaps the ssh child when
  the response ends so an aborted download cannot orphan it.
- remoteReadFile(): bounded read for file-content.

file-raw, file-content, file-preview and file-thumbnail now share one local/
remote target resolution. Guards keep their local strength: lexical pre-check,
remote realpath, workspace containment, sensitive-path blocklist, and the size
cap applied to the remote size before any bytes are read. An unreachable host
answers 502 with the remote reason instead of a misleading 404. Nothing is ever
copied to the Codeman host and there is NO local fallback (an sshfs mount of
the same tree must not shadow the remote bytes).

Deliberately unchanged: writes (edit=1 / PUT now answer 400 explicitly while
the viewer hides its Edit affordance), office previews, thumbnails, file tree,
picker, external attachment registration and tail-file stay local-only.

src/remote-hosts.ts

@@ -147,8 +147,14 @@ export function remoteSshTarget(host: Pick<RemoteHost, 'username' | 'host'>): st
  * POSIX single-quote shell-escaping (end-quote, escaped-quote, restart-quote).
  * Mirrors the helper in tmux-manager.ts so a value with spaces/metachars stays a
  * single shell token. Used here for identity paths and `-o KEY=VALUE` options.
+ *
+ * EXPORTED for `remote-files.ts` (#415, remote file access): that module wraps a
+ * remote shell command in the ssh line built by `buildSshConnectionArgs()`, so it
+ * needs the same escaping discipline for the remote command itself and for every
+ * path interpolated into it. A third private copy of this function is exactly how
+ * two escaping implementations drift apart.
  */
-function shellescape(str: string): string {
+export function shellescape(str: string): string {
   return "'" + str.replace(/'/g, "'\\''") + "'";
 }