fix(plugin): harden Windows path boundaries

[Mineru98]

Problem

Windows path forms can be normalized differently from Unix paths. Without consistent handling, plugin and config paths can look valid while still pointing outside the intended boundary.

Proposed change

• Normalize Windows-style paths before path boundary checks.
• Keep plugin entrypoints and config paths inside their expected directories.
• Update tests to cover portable path behavior across Windows and Unix-style inputs.

Acceptance criteria

• Windows drive, separator, and traversal cases are handled consistently.
• Plugin path validation rejects paths that escape the plugin boundary.
• Existing config and transport tests continue to pass.

Verification

• git diff --check
• mix format --check-formatted on changed Elixir files
• mix test test/ourocode/config/raw_loader_test.exs test/ourocode/plugin/path_policy_test.exs test/ourocode/command/registry/skill_loader_test.exs test/ourocode/mcp/transport/stdio_test.exs test/ourocode/mcp/transport/stdio_cleanup_test.exs test/ourocode/terminal/interview_panel/text_test.exs -> 42 passed

Closes #50