bug: mouse garbling after inactivity — missing SIGTSTP handler #20506

Description

When opencode is suspended via external SIGTSTP (e.g., code-server terminal management during inactivity, job control), mouse escape sequences appear as garbled text in the shell. The bottom of the TUI display gets cut off.

This is distinct from #20458 (post-exit garbling) and #3199 (in-session fragmentation).

Root Cause

Neither @opentui/core nor opencode register a SIGTSTP handler. When SIGTSTP arrives from an external source, the process is suspended by the kernel with zero cleanup:

• Mouse tracking remains enabled
• Raw mode remains on
• Shell receives mouse events as garbled escape sequences

The correct cleanup pattern already exists in the manual suspend keybind (app.tsx), which calls renderer.suspend() → disableMouse() → setRawMode(false) before suspending. But this only runs from the TUI command palette — an external SIGTSTP bypasses it.

Reproduction

# Terminal 1: run opencode
opencode

# Terminal 2: send SIGTSTP
kill -TSTP $(pgrep -f opencode)

# Terminal 1: move mouse → garbled text

Also reproducible in code-server when the terminal management layer suspends inactive processes.

Workaround

Register SIGTSTP/SIGCONT handlers in app.tsx that call renderer.suspend()/renderer.resume().

Upstream fix: anomalyco/opentui#907
Upstream issue: anomalyco/opentui#906

Environment

• code-server (VS Code web terminal) on VM
• Triggered by inactivity / external SIGTSTP