fix: leave a live socket alone instead of taking over its path at startup

[arzafran]

What this does

Launching a second build of Programa (a bench, a staging copy, a build with a different bundle id) no longer knocks the running app's command line tool and agent hooks offline. The new copy used to delete the running app's socket and take it over, and the running app stayed unreachable until someone ran Restart CLI Listener.

Do not merge until the fix commit is on this PR. The first push is the failing test alone, so CI shows it catches the bug.

Summary

• bindListenerSocket unlinked the socket path before binding without checking whether another app was listening there. fix: give windowless launches a window and stop them deleting another app's socket #426 stopped the same takeover on quit; this covers startup.
• It now probes the path first. Only a path proven stale or missing is unlinked. A live listener fails as EADDRINUSE, and an inconclusive probe (for example EMFILE, or a listener owned by another user) fails with its errno, so the default path falls back to the user-scoped one (programa-<uid>.sock) instead of taking the incumbent's path. The CLI resolves the default path first whenever it is live, so the fallback never redirects it.
• The bind-failure log now includes the path, stage and errno.

Test Plan

• CI: testStartLeavesALiveListenerFromAnotherProcessInPlace fails on the test commit and passes once the fix is pushed.
• Real apps: with production Programa running, launched a Release build with a different bundle id and no socket override. Production's socket kept the same inode and still answered ping, and the new build bound programa-501.sock.