fix: v2.5.7 — panes work on psmux, the Windows tmux

psmux (the WinGet `tmux` on Windows) has no `=name` exact-match targets: it
answered "no server running on session '=agenthub-lane-1'" for a live pane,
so every Windows start reported "the session's command exited as soon as it
started" while the session ran on, and the browser could not attach.

- server/lib/tmux.js: `session`/`pane` targets probe the first live session
  once — `=name` failing where the bare name answers means bare names from
  then on; real tmux keeps `=name` / `=name:` (it matches by prefix otherwise)
- pty-attach.js and cc-update-all-helper.js use the same targets

server/lib/pty-attach.js

@@ -70,7 +70,7 @@ function attach(ws, runId, { cols, rows }) {
   } catch {
     /* session gone or tmux unavailable — attach below will surface that */
   }
-  const pty = spawnImpl(attachBinary(), ["attach-session", "-t", `=${runId}`], {
+  const pty = spawnImpl(attachBinary(), ["attach-session", "-t", tmux.session(runId)], {
     name: "xterm-256color",
     cols: cols || 80,
     rows: rows || 24,