fix(statusline): unset a stale user statusline var, write the exporter script atomically

Three small follow-ups from the #361 review.

A tmux setenv survives respawn-pane, so _configureStatusLineUserCommand
returning early when the user has no statusline left a previously
exported CODEMAN_USER_STATUSLINE_CMD in place: a user who deleted their
own statusline kept getting the stale one wrapped, and lost Codeman's
footer print-through, until the tmux session was recreated. It now
issues `setenv -u` in that case, the same shape as the effort-level
cleanup in applyEnvOverrides.

ensureStatusLineExporterScript truncated and rewrote a script that live
sessions execute on every statusline render, and chmod'd it after the
write. It now writes a temp file next to the target, chmods that, and
rename()s it into place.

The non-tmux direct-PTY fallback carries no exporter; that is now stated
at the spawn site and in the architecture-invariants paragraph rather
than left as a silent gap.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/session.ts

@@ -2174,7 +2174,12 @@ export class Session extends EventEmitter {
       }
       try {
         // Pass --session-id to use the SAME ID as the Codeman session
-        // This ensures subagents can be directly matched to the correct tab
+        // This ensures subagents can be directly matched to the correct tab.
+        // No plan-usage statusLine exporter on this path: the ephemeral
+        // `--settings` injection (resolveStatusLineCliCommand, hooks-config.ts)
+        // is wired into the tmux spawn builders only, so a direct-PTY session
+        // has no Claude telemetry in the header chip. Documented in
+        // architecture-invariants (Plan-usage chip); tmux is the supported path.
         const args = buildInteractiveArgs(
           this.id,
           this._claudeMode,