Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

.github/workflows/ci.yml

@@ -55,13 +55,16 @@ jobs:
         uses: golangci/golangci-lint-action@v6
         with:
           version: v1.64
+          working-directory: backend
 
       # Vet
       - name: Run go vet
+        working-directory: backend
         run: go vet ./...
 
       # Unit + Integration Tests
       - name: Run unit & integration tests
+        working-directory: backend
         env:
           GOCHAT_ENV: test
           GOCHAT_TEST_DB: ${{ matrix.db-mode }}
@@ -78,6 +81,7 @@ jobs:
       # E2E Tests (only on postgres matrix)
       - name: Run e2e tests
         if: matrix.db-mode == 'postgres'
+        working-directory: backend
         env:
           GOCHAT_ENV: test
           POSTGRES_HOST: localhost
@@ -91,6 +95,7 @@ jobs:
 
       # Benchmark (quick sanity check, not full bench)
       - name: Run benchmarks
+        working-directory: backend
         env:
           GOCHAT_ENV: test
           GOCHAT_TEST_DB: ${{ matrix.db-mode }}
@@ -106,7 +111,7 @@ jobs:
         if: matrix.db-mode == 'postgres'
         uses: codecov/codecov-action@v4
         with:
-          files: coverage.out
+          files: backend/coverage.out
 
   # ---- Stage 2: Security Scan ----
   security:
@@ -124,9 +129,11 @@ jobs:
         uses: securego/gosec@master
         with:
           args: '-no-fail ./...'
+        working-directory: backend
 
       # Dependency vulnerability scan
       - name: Run govulncheck
+        working-directory: backend
         run: go install golang.org/x/vuln/cmd/govulncheck@latest && govulncheck ./...
 
       # Trivy filesystem scan
@@ -185,7 +192,7 @@ jobs:
         uses: docker/build-push-action@v5
         with:
           context: .
-          file: ./Dockerfile
+          file: ./deploy/docker/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
@@ -260,8 +267,8 @@ jobs:
       # Post-deploy health check
       - name: Health check
         run: |
-          chmod +x scripts/health_check.sh
-          GOCHAT_HOST=gochat-staging GOCHAT_PORT=3000 ./scripts/health_check.sh --full --timeout 30
+          chmod +x backend/scripts/health_check.sh
+          GOCHAT_HOST=gochat-staging GOCHAT_PORT=3000 ./backend/scripts/health_check.sh --full --timeout 30
 
   # ---- Stage 6: Deploy to Production ----
   deploy-production:
@@ -290,5 +297,5 @@ jobs:
       # Post-deploy health check
       - name: Health check
         run: |
-          chmod +x scripts/health_check.sh
-          GOCHAT_HOST=gochat-production GOCHAT_PORT=3000 ./scripts/health_check.sh --full --timeout 30
+          chmod +x backend/scripts/health_check.sh
+          GOCHAT_HOST=gochat-production GOCHAT_PORT=3000 ./backend/scripts/health_check.sh --full --timeout 30