Add in-memory shell snapshots to unified exec (#39957)

## What changed

- Add the under-development `shell_snapshot_v2` feature and advertise executor support through environment capabilities.
- Use executor-managed, in-memory snapshots for eligible direct `bash`, `zsh`, and `sh` login commands, while applying the configured shell environment policy and avoiding duplicate automatic startup-file effects.
- Keep file-backed snapshots available for user-shell commands and fall back to the existing execution path when in-memory snapshots are unsupported or inapplicable.

## Testing

- Cover policy filtering, snapshot reuse without snapshot files, automatic startup files, local and remote execution, and legacy user-shell snapshots.

GitOrigin-RevId: 4ad6cdf13824913ac8c393ba38b9844230677579

codex-rs/features/src/lib.rs

@@ -145,6 +145,8 @@ pub enum Feature {
     UseLegacyLandlock,
     /// Experimental shell snapshotting.
     ShellSnapshot,
+    /// Keep policy-filtered shell snapshots entirely in executor memory.
+    ShellSnapshotV2,
     /// Allow turns to start while selected executors are still starting.
     DeferredExecutor,
     /// Use the current working directory for turn diff display paths.
@@ -871,6 +873,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::ShellSnapshotV2,
+        key: "shell_snapshot_v2",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::DeferredExecutor,
         key: "deferred_executor",