memories-mcp: reject symlink traversal in local backend

[jif-oai]

Why

The local memories MCP backend only rejected symlinks after resolving the final path. That left room for scoped requests like skills/secret.md to walk through a symlinked ancestor directory and escape the configured memories root.

This change also makes missing scoped paths fail explicitly instead of looking like an empty list / search result or a NotFile read error.

What Changed

• walk each scoped path component in LocalMemoriesBackend::resolve_scoped_path and reject symlinked ancestors before accessing the target
• reject scoped paths that traverse through a non-directory intermediate component
• add a NotFound backend error for missing read, list, and search paths and map it through the MCP server error conversion
• add coverage for missing paths and symlinked ancestor directories in codex-rs/memories/mcp/src/local_tests.rs

Testing

• added unit coverage in codex-rs/memories/mcp/src/local_tests.rs for missing paths and symlinked ancestor directories across read, list, and search

[jif-oai]

@codex review

[chatgpt-codex-connector]

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

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