fix(tmux): restore Ctrl+Z suspend/resume inside attached sessions

[rhukster]

Summary

Ctrl+Z (suspend) has been broken inside agent-deck tmux sessions, leaving users in an unrecoverable frozen terminal state where control characters echo as raw text (^Z^Z^Z^Z). Only Ctrl+Q (detach) still worked. This PR fixes two independent root causes:

Problem 1: PTY line discipline suspending the tmux client

The PTY master fd connecting agent-deck to tmux attach-session had default terminal settings with ISIG enabled. When the user pressed Ctrl+Z, the PTY's line discipline interpreted byte 0x1A as SUSP and delivered SIGTSTP to the tmux attach process — suspending the tmux client. This left the user in a frozen limbo state: no process was actively reading input, so further control characters were simply echoed as text. Only Ctrl+Q escaped because it is intercepted in the stdin-reader goroutine before bytes reach the PTY.

Fix: Set the PTY master to raw mode (term.MakeRaw) immediately after creation, so all bytes pass through transparently to tmux. This is safe because:

• The PTY master fd is fully isolated from os.Stdin/os.Stdout and Bubble Tea's terminal state
• tmux expects to receive raw bytes and routes control characters to panes internally
• The existing Ctrl+Q interception in the stdin-reader goroutine is unaffected (it operates before bytes reach the PTY)

Problem 2: Non-interactive bash -c wrapper breaking job control

Compound session commands (containing $() subshells for UUID generation) are wrapped in bash -c '...' for fish shell compatibility. This non-interactive bash shell has no job control. When Claude Code handled Ctrl+Z internally by sending SIGTSTP to itself, the bash -c parent couldn't handle the stopped child — leaving the pane in a broken state.

Additionally, wrapIgnoreSuspend wrapped every command in bash -c 'stty susp undef; ...', which disabled the terminal's ability to generate SIGTSTP from Ctrl+Z.

Fix (two parts):

• Add exec before the final claude invocation in compound commands. This replaces the bash -c process with claude, making it a direct child of the interactive shell where job control works naturally (Ctrl+Z → [1]+ Stopped → shell prompt → fg to resume).
• Only apply wrapIgnoreSuspend for sandboxed sessions, where the command runs as the pane's initial process with no interactive shell for job control. Non-sandbox sessions launch via send-keys into the user's interactive shell, so standard job control works without intervention.

Test plan

• Create a new non-sandbox Claude session, attach, press Ctrl+Z — Claude should suspend and show a shell prompt
• Type fg — Claude should resume normally
• Ctrl+Q still detaches back to the session list
• Ctrl+C inside an attached session passes through to the running process (not intercepted by the PTY)
• Sandboxed sessions still function correctly (wrapIgnoreSuspend still applied)
• Fork and new-session-with-message flows work correctly with the exec prefix
• Fish shell users can still create/resume sessions (bash -c wrapping still applied)