feat: paginate MCP memories list

[jif-oai]

Why

Large memories trees do not fit well into a single MCP list response. This change makes the memories MCP server page list results so callers can continue walking the tree without overfetching or relying on ambiguous truncation.

What changed

• add an optional cursor input to the memories MCP list API and return next_cursor alongside truncated in the response
• paginate recursive local-memory traversal while preserving lexicographic path order across directories
• reject malformed and out-of-range cursors as invalid MCP requests
• update the server/schema wiring and add coverage for pagination, ordering, and cursor validation in memories/mcp/src/local_tests.rs

Testing

• cargo test -p codex-memories-mcp