feat: add directory scope enforcement to block out-of-scope file access

[ehsaniara]

Summary

• Adds allowed_directories policy that blocks requests containing file references outside configured directories
• Prevents LLM tools from scanning files beyond the intended project scope (e.g. ~/.ssh, /etc/passwd, ~/.aws/credentials)
• Scope check runs before deny pattern check, resolves relative paths and ../ traversals
• Desktop UI updated with an "Allowed Directories" section in the Policy tab
• 6 new test cases covering in-scope, out-of-scope, multi-dir, bypass, and path traversal

Test plan

• All existing tests pass (24 policy, full suite green)
• New scope tests cover: absolute paths, relative paths, ../ traversal, multiple allowed dirs, bypass mode, empty config
• Frontend builds successfully
• Production binary builds and runs