Fix towncrier fragment format: use flat <name>.<type>.md

The towncrier config uses the type's `directory` field as the type
identifier in filenames, NOT as subdirectories. Correct format:
  docs/changelog.d/<name>.<type>.md

NOT:
  docs/changelog.d/<type>/<name>.md

- Move fragments to root with type suffix
- Remove empty type subdirectories
- Fix CLAUDE.md instructions
- Fix tutorial examples in contributing.md and ai-assistance-guide.md

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

docs/tutorials/ai-assistance-guide.md

@@ -54,10 +54,10 @@ EOF
 
 Add a fragment for user-visible changes:
 ```bash
-echo "Description" > docs/changelog.d/feature/branch-name.md
+echo "Description" > docs/changelog.d/branch-name.feature.md
 ```
 
-Types (directory names): `feature`, `bugfix`, `infra`, `doc`, `misc`
+Types (file suffix): `.feature`, `.bugfix`, `.infra`, `.doc`, `.misc`
 
 ## Service Locations
 

docs/tutorials/contributing.md

@@ -95,15 +95,15 @@ Depending on what you're changing:
 
 For user-visible changes:
 ```bash
-echo "Description of your change" > docs/changelog.d/feature/your-branch.md
+echo "Description of your change" > docs/changelog.d/your-branch.feature.md
 ```
 
-Fragment types (use as directory name):
-- `feature/` - New functionality
-- `bugfix/` - Bug fixes
-- `infra/` - Infrastructure changes
-- `doc/` - Documentation
-- `misc/` - Other
+Fragment types (file suffix):
+- `.feature.md` - New functionality
+- `.bugfix.md` - Bug fixes
+- `.infra.md` - Infrastructure changes
+- `.doc.md` - Documentation
+- `.misc.md` - Other
 
 ### 4. Test Your Changes