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/core/config.schema.json

@@ -754,6 +754,9 @@
             "shell_snapshot": {
               "type": "boolean"
             },
+            "shell_snapshot_v2": {
+              "type": "boolean"
+            },
             "shell_tool": {
               "type": "boolean"
             },
@@ -5712,6 +5715,9 @@
         "shell_snapshot": {
           "type": "boolean"
         },
+        "shell_snapshot_v2": {
+          "type": "boolean"
+        },
         "shell_tool": {
           "type": "boolean"
         },