feat: add support for CODEX_SECURE_MODE=1 to restrict process observability (#4220)

Because the `codex` process could contain sensitive information in
memory, such as API keys, we add logic so that when
`CODEX_SECURE_MODE=1` is specified, we avail ourselves of whatever the
operating system provides to restrict observability/tampering, which
includes:

- disabling `ptrace(2)`, so it is not possible to attach to the process
with a debugger, such as `gdb`
- disabling core dumps

Admittedly, a user with root privileges can defeat these safeguards.

For now, we only add support for this in the `codex` multitool, but we
may ultimately want to support this in some of the smaller CLIs that are
buildable out of our Cargo workspace.

codex-rs/Cargo.toml

@@ -72,6 +72,7 @@ clap = "4"
 clap_complete = "4"
 color-eyre = "0.6.3"
 crossterm = "0.28.1"
+ctor = "0.5.0"
 derive_more = "2"
 diffy = "0.4.2"
 dirs = "6"