Fix shell injection in run_editor and add STDERR TTY check for --interactive

[Copilot]

Addresses two issues from code review:

• run_editor shell injection (src/cjules/unidiff.cr): shell_escape didn't correctly handle single quotes in POSIX shells due to gsub interpreting backslashes in the replacement string. Removed shell invocation entirely — editor command is now split into argv and the path is passed as a separate argument to Process.run, eliminating both the escaping bug and injection risk.

• Incomplete TTY check (src/cjules/commands/patch.cr): --interactive only validated STDIN.tty? && STDOUT.tty?, but prompts are written to STDERR. Added STDERR.tty? so the command errors out instead of silently hanging when STDERR is redirected.

# Before: shell invocation with broken quoting
cmd = "#{editor} #{shell_escape(path)}"
Process.run("sh", ["-lc", cmd], ...)

# After: direct exec, no shell
parts = editor.split
Process.run(parts[0], parts[1..] + [path], ...)