docs: add ruamel.yaml PEP 723 exception to ADR 0002

Document that prow skill scripts use ruamel.yaml via PEP 723 inline
script metadata for round-trip YAML fidelity. The dependency is managed
by uv run --script with no user-facing install step.

Addresses non-blocking review item from PR #25.

Assisted-by: OpenCode

Author: zdrapela

docs/adr/0002-stdlib-only-python-clis.md

@@ -9,3 +9,9 @@ Both CLIs (`rhdh` and `rhdh-local`) use only Python 3.9+ standard library — ze
 - **`uv`** as the dev tool runner (`uv run pytest`) — not shipped with the CLIs, but used for development and testing
 
 New scripts and CLI commands in this project should follow these same patterns.
+
+## Exceptions
+
+Prow skill scripts (under `skills/prow/scripts/`) use `ruamel.yaml` for round-trip YAML processing — preserving comments, key ordering, and quoting style that the stdlib `yaml` module cannot handle. These scripts declare the dependency via PEP 723 inline script metadata (`# /// script` blocks), and `uv run --script` auto-installs it in an ephemeral virtual environment. No user-facing install step is required.
+
+The stdlib-only rule still applies to the main CLIs and any script not run via `uv run --script`.