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/core/src/tools/runtimes/shell/unix_escalation.rs

@@ -126,6 +126,7 @@ pub(super) async fn try_run_zsh_fork(
         command,
         cwd: sandbox_cwd,
         env: sandbox_env,
+        exec_server_env_config: _,
         network: sandbox_network,
         expiration: _sandbox_expiration,
         capture_policy: _capture_policy,
@@ -734,6 +735,7 @@ impl ShellCommandExecutor for CoreShellCommandExecutor {
                 command: self.command.clone(),
                 cwd: self.cwd.clone(),
                 env: exec_env,
+                exec_server_env_config: None,
                 network: self.network.clone(),
                 expiration: ExecExpiration::Cancellation(cancel_rx),
                 capture_policy: ExecCapturePolicy::ShellTool,