chore: move memory prompt builder into extension

[jif-oai]

Why

The memories extension now owns the read-path developer instructions it injects at thread start. Keeping that prompt builder and template in codex-memories-read left the extension depending on a helper crate for extension-specific prompt assembly, and kept async template/truncation dependencies in the read crate after the remaining read surface no longer needed them.

What changed

• Moved prompts.rs, its tests, and templates/memories/read_path.md from memories/read into ext/memories.
• Wired MemoryExtension to call the local prompt builder and added the moved templates to ext/memories/BUILD.bazel compile data.
• Removed the now-unused prompt export and prompt-related dependencies from codex-memories-read.

Testing

• Not run locally.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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