[codex] add path-types skill (#28347)

## Why

Codex contributors and agents need repository-scoped guidance for
choosing compatible Rust types
for operating system paths during the ongoing URI migration. Keeping the
guidance in the repository
makes the app-server and exec-server rules available consistently
without relying on a personal
skill installation.

## What

- Add the `path-types` skill at `.codex/skills/path-types/SKILL.md`.
- Document the intended uses of `ApiPathString`, `PathUri`,
`AbsolutePathBuf`, and `PathBuf` across
  protocol, internal, and shared dependency boundaries.
- Keep migrations of existing types limited to explicit requests and
proportional edits.

## Validation

- Validated the skill structure with skill-creator's
`quick_validate.py`.

.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.