fix: make peek after kill deterministically report no session

[kylecarbs]

Fixes the flaky agent loop: new, send, wait, peek, kill integration test that failed on macOS CI (run 27382774714) with boo peek: wanted exit 3, got .{ .Exited = 1 }: error: ConnectionLost.

Problem

The daemon acked quit while its socket file and listener still existed; the socket was unlinked only later during teardown, after Window.destroy reaped the child. A peek issued right after kill returned could connect in that window, read EOF, and exit 1 with error.ConnectionLost instead of the documented no-session exit 3.

Fix

• daemon: retire the listener (close it and unlink the socket file) before acking quit. By the time kill returns, no new client can find the socket, making the sequential kill-then-peek case deterministic.
• client: map error.ConnectionLost in mustControl to the no-session failure (exit 3). An EOF on a control exchange means the daemon died before replying, which also covers control connections racing a natural command exit.
• test: regression test repeating the kill-then-peek sequence 10 times.

Verification

A/B with a temporary 100ms sleep at the top of daemon teardown to widen the race window, driving a 30-iteration new/kill/peek loop:

build	result
main + 100ms window	30/30 failed, error: ConnectionLost, exit 1 (exact CI failure)
this PR + 100ms window	0/30 failed

Also zig build test-all: 132/132 passed, and a 300-iteration stress loop on the release build: 0 failures.

Investigation notes

• sessionInfo already treats any control error as "daemon gone", so ls and wait --idle were robust; mustControl callers (send, peek, wait --text, kill, rename) were not.
• The window was wide enough to hit on loaded macOS runners because deinit runs Window.destroy (child reaping) before unlinking the socket.
• Daemon.deinit now reuses retireListener, which guards against double-close after an early retire.
• The race did not reproduce naturally on a fast Linux box (0/300 on main), hence the deterministic widened-window A/B.

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This PR was generated by Coder Agents on behalf of @kylecarbs.