[codex] add path-types skill

.codex/skills/path-types/SKILL.md

@@ -0,0 +1,18 @@
+---
+name: path-types
+description: Choose Rust types for operating system paths across the Codex repository. Use when defining new path-bearing types or explicitly migrating existing ones.
+---
+
+# Path Types
+
+Apply this guidance when defining new types. Change existing code only when explicitly requested,
+and keep edits minimal and proportional. Treat these rules as the target state of an ongoing
+migration; if compliance is difficult, ask the user how to proceed.
+
+- In app-server protocol types, use `ApiPathString` for backwards compatibility during the URI
+  migration. At the protocol boundary, convert it to `PathUri` and use `PathUri` internally. For
+  host-local logic, such as some config values, use `AbsolutePathBuf` or `PathBuf` instead.
+- In exec-server protocol types, use `PathUri`. Internally, use `PathUri` or `AbsolutePathBuf` as
+  appropriate.
+- In dependencies shared by both servers, use `PathUri` or separate APIs that decouple their use
+  cases.