fix: keep prompt out of argv when promptViaStdin/ACP is enabled

On Linux, spawning the agent with a long prompt appended to argv exceeds
the kernel ARG_MAX and fails with spawn E2BIG. Setting
CURSOR_BRIDGE_PROMPT_VIA_STDIN=true did not help: both handlers still set
cmdArgs = fit.args, which appends the full prompt to argv (fitPromptToWinCmdline
only guards the Windows command-line limit, not Linux ARG_MAX). The prompt was
therefore sent via BOTH argv and stdin, so argv still overflowed.

Now, when promptViaStdin or ACP is active, cmdArgs uses fixedArgs (no prompt in
argv); the prompt is delivered via stdin only. This is the Linux counterpart to
the Windows ENAMETOOLONG fix from #13.

Adds regression tests asserting the prompt is passed via stdin (not argv) when
promptViaStdin is true, and via argv (not stdin) when it is false.