chore: add VSCode settings and update README for clarity**

### CHANGES
- Add recommended extensions for VSCode in `extensions.json`
- Create `settings.json` for custom spell-checker words
- Update README to specify plaintext in code block

.vscode/extensions.json

@@ -0,0 +1,6 @@
+{
+	"recommendations": [
+		"davidanson.vscode-markdownlint",
+		"streetsidesoftware.code-spell-checker"
+	]
+}

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+	"cSpell.words": ["BMAD", "roadmaps"]
+}

README.md

@@ -70,7 +70,7 @@ npm run validate       # Validate configurations
 
 ## Project Structure
 
-```
+```plaintext
 bmad-core/
 ├── agents/          # Individual agent definitions
 ├── agent-teams/     # Team configurations