[codex] Add remote thread store implementation (#17826)

- Add a "remote" thread store implementation
- Implement the remote thread store as a thin wrapper that makes grpc
calls to a configurable service endpoint
- Implement only the thread/list method to start
- Encode the grpc method/param shape as protobufs in the remote
implementation

A wart: the proto generation script is an "example" binary target. This
is an example target only because Cargo lets examples use
dev-dependencies, which keeps tonic-prost-build out of the normal
codex-thread-store dependency surface. A regular bin would either need
to add proto generation deps as normal runtime deps, or use a
feature-gated optional dep, which this repo’s manifest checks explicitly
reject.

codex-rs/thread-store/scripts/generate-proto.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+repo_root="$(cd "$script_dir/../../.." && pwd)"
+proto_dir="$repo_root/codex-rs/thread-store/src/remote/proto"
+generated="$proto_dir/codex.thread_store.v1.rs"
+tmpdir="$(mktemp -d)"
+
+cleanup() {
+    rm -rf "$tmpdir"
+}
+trap cleanup EXIT
+
+(
+    cd "$repo_root/codex-rs"
+    CARGO_TARGET_DIR="$tmpdir/target" cargo run \
+        -p codex-thread-store \
+        --example generate-proto \
+        -- "$proto_dir"
+)
+
+if ! sed -n '2p' "$generated" | grep -q 'clippy::trivially_copy_pass_by_ref'; then
+    {
+        sed -n '1p' "$generated"
+        printf '#![allow(clippy::trivially_copy_pass_by_ref)]\n'
+        sed '1d' "$generated"
+    } > "$tmpdir/generated.rs"
+    mv "$tmpdir/generated.rs" "$generated"
+fi
+
+rustfmt --edition 2024 "$generated"
+
+awk '
+    NR == 3 && previous ~ /clippy::trivially_copy_pass_by_ref/ && $0 != "" { print "" }
+    { print; previous = $0 }
+' "$generated" > "$tmpdir/formatted.rs"
+mv "$tmpdir/formatted.rs" "$generated"