[Bug] macOS arm64: Second background agent spawn fails with "fork failed: Device not configured" in 2.1.207

Bug Description
Regression in 2.1.207 (macOS arm64): spawning a second background agent in a session fails; the first works. Error:

Failed to send command to pane %2: respawn pane failed: fork failed: Device not configured

Every subsequent attempt fails (pane index increments). A session restart allows exactly one more agent, then fails again. Works normally on 2.1.205.

Not an OS/tmux/resource issue — isolated it: a fresh non-Claude tmux server created 40 panes cleanly, plain fork made 40 processes cleanly, 190/511 ptys in use, ~567/5333 procs, disk fine. On 2.1.205, three background agents spawned clean on the same machine at the same moment. Only the .207 claude-swarm-<pid> server fails to allocate a second pane. Reproduces in both Ghostty and iTerm2. .207 was installed the morning it broke.

Impact: one background agent per session on .207; any implementer+reviewer, parallel, or team workflow is blocked. Workaround: pin to 2.1.205 + DISABLE_AUTOUPDATER=1.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.207
• Feedback ID: ec30c30f-2673-4846-84f1-1ab464138478

Errors

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