fix(edit-history): remote lookup finds tmux, a timeout reaches ssh, docs follow

Applies the review of #766.

The live lookup ran tmux on the bare PATH of the ssh command. On a host
where tmux sits in ~/.local/bin or /opt/homebrew/bin it found no pane, the
2>/dev/null hid that, and the panel stayed on the launch conversation after
a /clear. It now goes through withRemoteTmuxPath(), inside the sh script and
scoped to the $(...), so the other tools keep the PATH they had. The fake
tmux of the test block moved to $HOME/.local/bin for the same reason: that
directory is first on the new PATH, so a real tmux is never asked.

The two ssh commands get exec. Node signals the local sh -c, and dash stays
the parent of a plain command, so ssh survived the timeout. A test with a
fake ssh shows the process dies now and survives without exec.

The comment at the meta budget claimed a meta past it comes with a later
read. A meta comes only with a subagent's first read, so it is never sent.

Docs: the store is named as the exception to "never copied to local disk",
remoteProbePaths and the remote transcript lookup are described, and the
line saying a remote session loads no store is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/remote-sessions.md

@@ -420,7 +420,8 @@ still has it.
 **Not available over ssh (by choice, not by accident):** editing a file (writes would
 need SFTP; `docs/file-viewer-edit-plan.md` §6), office-document previews and
 generated thumbnails (both need the bytes on the server's disk — no remote file is ever
-spilled onto the server), the file-tree/picker listings, and `tail-file`. Those routes
+spilled onto the server, the Edit history store's diffs being the one exception), the
+file-tree/picker listings, and `tail-file`. Those routes
 are still local-only, so with an `sshfs` mount in place they read the mounted copy —
 the two views can only disagree when that mount is stale. Docker cases are unaffected:
 their workspace is bind-mounted at the same absolute path, so local `fs` reads real bytes.