Add external agent memory migration (#33444)

## What changed

- Add a feature-gated `MEMORY` migration item to the app-server protocol and TUI.
- Discover project memory Markdown files, preserve their project scope, and copy selected projects into the Codex memory extension workspace.
- Detect changed, renamed, and removed source projects, report per-project import failures, and enqueue memory consolidation after workspace changes.

## Testing

- Cover memory discovery, selection, synchronization, scope validation, feature gating, protocol results, and TUI rendering.

GitOrigin-RevId: 9b17b2e9a639a064dba8aaffbec6345ae419018e

codex-rs/external-agent-migration/src/lib.rs

@@ -2,8 +2,11 @@
 
 mod hooks_cla;
 mod hooks_cur;
+mod memory;
 pub mod sessions;
 
+pub use memory::ExternalMemoryFile;
+pub use memory::discover_external_memory_files;
 use serde_json::Value as JsonValue;
 use serde_yaml::Value as YamlValue;
 use std::collections::BTreeMap;