refactor(rules): point discourse-cli invocation at centralized creds store

Merge discourse-cli into tooling-cli/discourse and drop the
discourse-cli/.env source path from the rules. The canonical
invocation now uses ~/.creds/discourse.env (system-independent)
with the registry image. Marks Q2 (bin/ wrapper gap) resolved.

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Assisted-by: Crush:glm-5.2

AGENTS.md

@@ -71,8 +71,8 @@ docker run --rm --env-file ~/projects/KNEL-AIMiddleware/redmine-cli/.env \
     git.knownelement.com/reachableceo/redmine-cli:latest <command>
 # subcommands: whoami, projects, statuses, list, show <id>, create, update <id>, close <id>
 
-# discourse-cli  (source: ~/projects/KNEL-AIMiddleware/discourse-cli/)
-docker run --rm --env-file ~/projects/KNEL-AIMiddleware/discourse-cli/.env \
+# discourse-cli
+docker run --rm --env-file ~/.creds/discourse.env \
     git.knownelement.com/reachableceo/discourse-cli:latest <command>
 # subcommands: whoami, categories, cat-info, topics, show <id>, create, reply <id>, update <post_id>, delete <post_id>, search
 ```