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.

.github/skills/docker-stop-dev-scripts/run.sh

@@ -18,4 +18,4 @@ REPO_ROOT="$(cd "$SCRIPT_DIR/../../.." && pwd)"
 cd "$REPO_ROOT"
 
 # Stop development environment with Docker Compose
-exec docker compose -f docker-compose.dev.yml down
+exec docker compose -f .docker/compose/docker-compose.dev.yml down