fix(openclaw): add comment clarifying proc.exited race condition avoidance

cubic identified potential race condition where Bun's proc.exitCode
may be null immediately after stdout closes. Added clarifying
comment that await proc.exited ensures exitCode is set before
checking.

fixes: cubic review on PR #2620

src/openclaw/tmux.ts

@@ -17,6 +17,7 @@ export async function getTmuxSessionName(): Promise<string | null> {
     const outputPromise = new Response(proc.stdout).text()
     await proc.exited
     const output = await outputPromise
+    // Await proc.exited ensures exitCode is set; avoid race condition
     if (proc.exitCode !== 0) return null
     return output.trim() || null
   } catch {