fix(sidecar): route shared-terminal child output through the PTY master only

[NathanFlurry]

• A child process sharing the shell's terminal (its kernel fd 1 is the inherited PTY slave) used to surface its output twice: once relayed by the shell runner from the child's stdout events, and once when the same bytes — written into the PTY slave — were drained from the master on the next input write. Every external command's output printed twice, and vim's per-keystroke echo corrupted the screen (stray characters where vim skipped repainting a cell it knew was blank).
• Children now record the (pid, master fd) owner of the shared terminal at spawn. Their stdio writes go through their own PTY slave (line discipline applies) and the master is drained immediately, surfacing the bytes once as the owning shell process's ordered output stream — no child stdout event is queued, so nothing gets relayed, matching native semantics where a shell never sees its child's tty output.
• Applies to both the child-event pump (foreground children) and the main sync-RPC handler (detached children), and propagates through nested spawns.