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/.env.example

@@ -0,0 +1,5 @@
+# Discourse connection details
+# Copy this file to .env and fill in real values. The .env file is gitignored.
+DISCOURSE_URL=https://discourse.example.com
+DISCOURSE_API_KEY=your-api-key-here
+DISCOURSE_API_USERNAME=your-username