refactor(tooling): merge discourse-cli into tooling-cli/discourse

Consolidate the discourse-cli source into tooling-cli/discourse/
(CLI source, Dockerfile, README, AGENTS.md, validate.sh) with all
documentation rewritten to invoke the container via raw docker run
and credentials from the centralized ~/.creds/discourse.env store.
No bin/ wrapper, no system-dependent paths in the docs.

Removes the old discourse-cli/ subdirectory and updates the
KNELCredsManager consumer table to reference the container directly.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

tooling-cli/discourse/Dockerfile

@@ -0,0 +1,16 @@
+FROM python:3.12-slim
+
+ENV PYTHONUNBUFFERED=1 \
+    PYTHONDONTWRITEBYTECODE=1 \
+    PIP_NO_CACHE_DIR=1
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install -r requirements.txt
+
+COPY src/discourse_cli.py /usr/local/bin/discourse-cli
+RUN chmod +x /usr/local/bin/discourse-cli
+
+ENTRYPOINT ["discourse-cli"]
+CMD ["--help"]