Relocate mcp.rs + mcp/* + mcp_tests.rs out of openpencil-shell-core
into a dedicated op-mcp crate. The MCP tool registry + JSON-RPC stdio
layer only depends on jian-ops-schema + op-editor-core (its tools
build on EditorState / EditorCommand). mcp.rs becomes the crate's
lib.rs; the mcp/ submodule files flatten to src/ so the `pub mod`
declarations resolve unchanged, and intra-module `super::` paths stay
valid. mcp_tests.rs becomes a #[cfg(test)] mod of lib.rs with its
`super::mcp::` paths rewritten to `crate::`. openpencil-desktop's
mcp_serve.rs now imports `op_mcp::*`. Pure relocation, no behaviour
change; 138 tests move with their code.
