cmux: tmux subagent panes never attach (respawn-pane no-op) — need eager split-attach

[Yoonkeee]

Problem

When running OmO inside cmux (the native-split Terminal layer), tmux subagent panes are created with a placeholder message ("Focus this pane to attach.") but they never transition to an actual opencode attach session, even when the user focuses the pane.

There are two compounding failure modes:

1. respawn-pane -k is a no-op under cmux __tmux-compat — The activation path in activateFocusedPanes calls activateTmuxPane, which issues a tmux respawn-pane -k <pane_id> command to replace the placeholder with opencode attach. Under cmux, cmux __tmux-compat intercepts all tmux commands and silently discards respawn-pane -k (it has no equivalent native-split operation), so the pane stays on the placeholder forever.

2. Immediate child-exit on focus — In some cmux configurations, focusing the placeholder pane causes the /bin/sh sleep loop to receive a signal or the pane's shell to terminate, killing the placeholder before any activation occurs. The subagent pane disappears on focus.

Root Cause

The placeholder + focus-to-activate design (buildTmuxPlaceholderCommand + activateTmuxPane respawn) assumes a real tmux server that can reuse an existing pane with respawn-pane -k. cmux emulates tmux's split-window for pane creation but does not implement respawn-pane -k. Consequently:

• The pane is created (split-window works ✓)
• The placeholder loop runs (✓)
• respawn-pane -k to inject opencode attach is silently dropped (✗)
• The pane remains stuck on the placeholder indefinitely

Proposed Fix

When isCmuxCompatEnvironment() returns true (already exists in packages/tmux-core/src/cmux-detect.ts), skip the placeholder entirely and pass buildTmuxAttachCommand(serverUrl, sessionId, directory) as the initial command to split-window. Simultaneously mark the spawned tracked session with attachActivated: true so the existing activateFocusedPanes guard (if (tracked.attachActivated) continue) skips the respawn attempt automatically.

This is an opt-in, cmux-only path — standard tmux behavior (focus-defer) is unchanged.

Impact

All OmO users running cmux (TMUX contains "cmuxterm" OR CMUX_SOCKET_PATH is set without TMUX) see subagent panes that appear to spawn but never become interactive. The feature is entirely non-functional in this environment.

Environment

• cmux: 0.64.x (cmuxterm native-split mode)
• OmO: dev branch
• Platform: macOS (Apple Silicon), other platforms untested

Relates To

• [Bug]: tmux subagent panes are created but stay blank with no visible execution output #3280 (tmux subagent core)
• [Bug]: Tmux subagent pane flash-closes on focus when directory path contains spaces #5596 (pane command path/space handling)
• [Feature]: Eager-attach option for tmux subagent panes (why is on-focus attach the default?) #5746 (session lifecycle)
• #6244 (cmux compatibility)