feat(network-proxy): add MITM support and gate limited-mode CONNECT (#9859)

## Description
- Adds MITM support (CA load/issue, TLS termination, optional body
inspection).
- Adds `codex-network-proxy init` to create
`CODEX_HOME/network_proxy/mitm`.
- Enforces limited-mode HTTPS correctly: `CONNECT` requires MITM,
otherwise blocked with `mitm_required`.
- Keeps `origin/main` layering/reload semantics (managed layers included
in reload checks).
- Centralizes block reasons (`REASON_MITM_REQUIRED`) and removes
`println!`.
- Scope is MITM-only (no SOCKS changes).

gated by `mitm=false` (default)

codex-rs/network-proxy/src/lib.rs

@@ -1,8 +1,10 @@
 #![deny(clippy::print_stdout, clippy::print_stderr)]
 
 mod admin;
+mod certs;
 mod config;
 mod http_proxy;
+mod mitm;
 mod network_policy;
 mod policy;
 mod proxy;