## Why
[#18763](https://github.com/openai/codex/pull/18763) added canonical
hostname resolution for `remote_sandbox_config`. Requirements
composition currently performs that synchronous DNS lookup on every
fresh process, even when none of the loaded requirements layers contains
`[[remote_sandbox_config]]`. On hosts with slow local DNS resolution,
this can add several seconds to Codex startup.
## What
- defer hostname resolution until a parsed requirements layer actually
contains `remote_sandbox_config`
- cache the resolver result once per requirements composition,
preserving the existing single-lookup behavior across multiple layers
- keep the existing FQDN resolution and per-layer requirements
precedence unchanged
- cover both the ordinary no-lookup path and the multi-layer
single-lookup path
## How to Test
On a host where local canonical-name resolution is slow:
1. Start Codex without `[[remote_sandbox_config]]` in any managed
requirements layer and confirm startup no longer waits for hostname
resolution.
2. Add a matching `[[remote_sandbox_config]]` entry and confirm its
`allowed_sandbox_modes` still overrides the layer's top-level value.
3. Add remote sandbox entries to multiple requirements layers and
confirm precedence remains unchanged while the hostname is resolved only
once.
Targeted tests:
- `just test -p codex-config hostname_resolver`
- `just test -p codex-config` (181 passed)
Felipe CouryandGitHub
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