refactor: split memories extension crate modules

[jif-oai]

Why

The memories extension has several distinct responsibilities: registering its prompt and tool contributors, enforcing local-memory filesystem boundaries, implementing list/read/search behavior, and wrapping that backend as extension tools. Those responsibilities were concentrated in lib.rs, local.rs, and the tool modules, which made follow-up work harder to review and risked growing files through unrelated edits.

This PR reorganizes the crate so each responsibility has a narrower owner while preserving the same extension entrypoint and memory tool behavior.

What Changed

• Moved extension lifecycle, prompt, and tool registration into src/extension.rs, leaving src/lib.rs as the small crate entrypoint.
• Split LocalMemoriesBackend helpers into local/list.rs, local/path.rs, local/read.rs, and local/search.rs.
• Centralized tool names and limits at the crate level, and kept the backend and extension implementation crate-private.
• Made memory_list, memory_read, and memory_search tool executors generic over MemoriesBackend, so tests can exercise the full executor path without depending on tool internals.
• Consolidated and expanded memory extension tests in src/tests.rs, including read/search tool output coverage, multi-query search, windowed all_within_lines, and legacy query rejection.

Testing

• Not run locally.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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