chore: upgrade BMAD framework to skill-based architecture

Migrates from slash-command files (.claude/commands, .cursor/commands)
to the new skill-based architecture with workflow step files.

_bmad/tea/workflows/testarch/atdd/workflow-plan.md

@@ -1,21 +0,0 @@
-    # Workflow Plan: testarch-atdd
-
-    ## Create Mode (steps-c)
-    - step-01-preflight-and-context.md
-
-- step-02-generation-mode.md
-- step-03-test-strategy.md
-- step-04-generate-tests.md
-- step-05-validate-and-complete.md
-
-  ## Validate Mode (steps-v)
-  - step-01-validate.md
-
-  ## Edit Mode (steps-e)
-  - step-01-assess.md
-  - step-02-apply-edit.md
-
-  ## Outputs
-  - {test_artifacts}/atdd-checklist-{story_id}.md
-
-- Failing acceptance tests under {project-root}/tests