Build remote exec env from exec-server policy (#17216)

## Summary
- add an exec-server `envPolicy` field; when present, the server starts
from its own process env and applies the shell environment policy there
- keep `env` as the exact environment for local/embedded starts, but
make it an overlay for remote unified-exec starts
- move the shell-environment-policy builder into `codex-config` so Core
and exec-server share the inherit/filter/set/include behavior
- overlay only runtime/sandbox/network deltas from Core onto the
exec-server-derived env

## Why
Remote unified exec was materializing the shell env inside Core and
forwarding the whole map to exec-server, so remote processes could
inherit the orchestrator machine's `HOME`, `PATH`, etc. This keeps the
base env on the executor while preserving Core-owned runtime additions
like `CODEX_THREAD_ID`, unified-exec defaults, network proxy env, and
sandbox marker env.

## Validation
- `just fmt`
- `git diff --check`
- `cargo test -p codex-exec-server --lib`
- `cargo test -p codex-core --lib unified_exec::process_manager::tests`
- `cargo test -p codex-core --lib exec_env::tests`
- `cargo test -p codex-core --lib exec_env_tests` (compile-only; filter
matched 0 tests)
- `cargo test -p codex-config --lib shell_environment` (compile-only;
filter matched 0 tests)
- `just bazel-lock-update`

## Known local validation issue
- `just bazel-lock-check` is not runnable in this checkout: it invokes
`./scripts/check-module-bazel-lock.sh`, which is missing.

---------

Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: pakrym-oai <pakrym@openai.com>

codex-rs/exec-server/src/protocol.rs

@@ -3,6 +3,7 @@ use std::path::PathBuf;
 
 use crate::FileSystemSandboxContext;
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
+use codex_config::types::ShellEnvironmentPolicyInherit;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use serde::Deserialize;
 use serde::Serialize;
@@ -64,11 +65,23 @@ pub struct ExecParams {
     pub process_id: ProcessId,
     pub argv: Vec<String>,
     pub cwd: PathBuf,
+    #[serde(default)]
+    pub env_policy: Option<ExecEnvPolicy>,
     pub env: HashMap<String, String>,
     pub tty: bool,
     pub arg0: Option<String>,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ExecEnvPolicy {
+    pub inherit: ShellEnvironmentPolicyInherit,
+    pub ignore_default_excludes: bool,
+    pub exclude: Vec<String>,
+    pub r#set: HashMap<String, String>,
+    pub include_only: Vec<String>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct ExecResponse {