## Why
Thread-scoped config needs a stable boundary between the app/session
owner and the config stack. Instead of having call sites manually copy
thread config fields into individual overrides, this adds the proto and
Rust plumbing needed for a `ThreadConfigLoader` implementation to return
typed sources that can be translated into ordinary config layer entries.
Keeping the remote payload typed also makes precedence easier to reason
about: session-owned thread config maps back to the existing session
config source, while user-owned thread config is represented separately
without introducing a new config-layer source until it has TOML-backed
fields.
## What changed
- Added the `codex.thread_config.v1` protobuf service and generated Rust
module for loading thread config sources.
- Added `RemoteThreadConfigLoader`, which calls the gRPC service, parses
`SessionThreadConfig` / `UserThreadConfig`, and validates provider
fields such as `wire_api`, auth timeout, and absolute auth cwd.
- Added proto generation tooling under
`config/scripts/generate-proto.sh` and
`config/examples/generate-proto.rs`.
- Added `ThreadConfigLoader::load_config_layers`, plus static/no-op
loader helpers, so tests and callers can use the same typed loader
interface while config-layer translation stays centralized.
## Verification
- `cargo test -p codex-config thread_config`
Rasmus Rygaard