chore: remove BMAD framework files and IDE configuration artifacts

Clean up unused BMAD workflow, agent, and command files across all IDE
configurations (.agent, .clinerules, .cursor, .gemini, .github, .kilocode,
.opencode) and internal module files (_bmad/bmb, _bmad/bmm).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/skills/bmad-product-brief/bmad-manifest.json

@@ -0,0 +1,17 @@
+{
+  "module-code": "bmm",
+  "replaces-skill": "bmad-create-product-brief",
+  "capabilities": [
+    {
+      "name": "create-brief",
+      "menu-code": "CB",
+      "description": "Produces executive product brief and optional LLM distillate for PRD input.",
+      "supports-headless": true,
+      "phase-name": "1-analysis",
+      "after": ["brainstorming, perform-research"],
+      "before": ["create-prd"],
+      "is-required": true,
+      "output-location": "{planning_artifacts}"
+    }
+  ]
+}