chore: reorganize repository structure

- Move docker-compose files to .docker/compose/
- Move docker-entrypoint.sh to .docker/
- Move DOCKER.md to .docker/README.md
- Move 16 implementation docs to docs/implementation/
- Delete test artifacts (block_test.txt, caddy_*.json)
- Update all references in Dockerfile, Makefile, tasks, scripts
- Add .github/instructions/structure.instructions.md for enforcement
- Update CHANGELOG.md

Root level reduced from 81 items to ~35 visible items.

.vscode/tasks.json

@@ -173,21 +173,21 @@
         {
             "label": "Docker: Start Local Environment",
             "type": "shell",
-            "command": "docker compose -f docker-compose.local.yml up -d",
+            "command": "docker compose -f .docker/compose/docker-compose.local.yml up -d",
             "group": "none",
             "problemMatcher": []
         },
         {
             "label": "Docker: Stop Local Environment",
             "type": "shell",
-            "command": "docker compose -f docker-compose.local.yml down",
+            "command": "docker compose -f .docker/compose/docker-compose.local.yml down",
             "group": "none",
             "problemMatcher": []
         },
         {
             "label": "Docker: View Logs",
             "type": "shell",
-            "command": "docker compose logs -f",
+            "command": "docker compose -f .docker/compose/docker-compose.yml logs -f",
             "group": "none",
             "problemMatcher": [],
             "isBackground": true