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/hooks-config.ts

@@ -1023,8 +1023,14 @@ export async function ensureStatusLineExporterScript(): Promise<string> {
     // Doesn't exist yet.
   }
   if (current !== desired) {
-    await writeFile(scriptPath, desired);
-    await chmod(scriptPath, 0o755);
+    // Temp file + rename: live sessions execute this script on every statusline
+    // render, and a truncate-then-write (plus a chmod AFTER the write) opened two
+    // windows in which Claude Code could run an empty or non-executable file.
+    // rename() swaps the complete, already-executable file in atomically.
+    const tmpPath = `${scriptPath}.${process.pid}.${Date.now()}.tmp`;
+    await writeFile(tmpPath, desired);
+    await chmod(tmpPath, 0o755);
+    await rename(tmpPath, scriptPath);
   }
   return scriptPath;
 }

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,

src/tmux-manager.ts

@@ -1850,16 +1850,24 @@ export class TmuxManager extends EventEmitter implements TerminalMultiplexer {
    * same reasoning that made the exporter script itself necessary (see
    * ensureStatusLineExporterScript's doc comment). Only this ONE line needs
    * shellescape(); the stored value itself is opaque to tmux from then on.
+   *
+   * With NO user command the variable is UNSET rather than left alone: a tmux
+   * setenv survives respawn-pane, so a user who deleted their own statusline
+   * would otherwise keep getting the stale one wrapped (and lose Codeman's
+   * footer print-through) until the tmux session was recreated. Same shape as
+   * the CLAUDE_CODE_EFFORT_LEVEL cleanup in applyEnvOverrides.
    */
   private _configureStatusLineUserCommand(muxName: string, command: string | undefined): void {
-    if (!command) return;
+    const setOrUnset = command
+      ? `CODEMAN_USER_STATUSLINE_CMD ${shellescape(command)}`
+      : '-u CODEMAN_USER_STATUSLINE_CMD';
     try {
-      execSync(`${this.tmux()} setenv -t ${shellescape(muxName)} CODEMAN_USER_STATUSLINE_CMD ${shellescape(command)}`, {
+      execSync(`${this.tmux()} setenv -t ${shellescape(muxName)} ${setOrUnset}`, {
         timeout: EXEC_TIMEOUT_MS,
         stdio: 'ignore',
       });
     } catch {
-      // Non-critical — the exporter just falls back to the plain "codeman" marker.
+      // Non-critical: the exporter prints its own footer, or nothing.
     }
   }