docs: add comprehensive Build Architecture Guard documentation

- Update main README with guard system overview
- Create detailed guard implementation guide
- Add PR template documentation and usage examples
- Document opt-in hook activation process

README-PR-TEMPLATE.md

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+# Pull Request Template
+
+## Location
+
+The Build Architecture Guard PR template is located at:
+
+- **`pull_request_template.md`** (root directory)
+
+## Usage
+
+When creating a pull request in Gitea, this template will automatically populate the PR description with the required checklist.
+
+## Template Features
+
+### Change Level Classification
+
+- **L1**: Minor changes, documentation updates
+- **L2**: Moderate changes, new features, environment changes
+- **L3**: Major changes, architecture changes, new platforms
+
+### Required Fields for All Levels
+
+- Change level selection
+- Scope and impact description
+- Commands executed and their output
+- Documentation updates (BUILDING.md)
+- Rollback verification steps
+
+### Additional Requirements for L3
+
+- **ADR link**: Must provide URL to Architectural Decision Record
+- **Artifacts with SHA256**: Must list artifacts with cryptographic hashes
+
+## Integration
+
+This template works with:
+
+- **Gitea Actions**: `.gitea/workflows/build-guard.yml`
+- **Client-side hooks**: `.husky/` pre-commit and pre-push hooks
+- **Guard script**: `scripts/build-arch-guard.sh`
+
+## Example Usage
+
+```markdown
+### Change Level
+- [x] Level: **L2**
+
+**Why:** Adding new build script for Docker deployment
+
+### Scope & Impact
+- [x] Files & platforms touched: scripts/build-docker.sh,
+  BUILDING.md
+- [x] Risk triggers: Docker build process changes
+- [x] Mitigations/validation done: Tested on local Docker environment
+
+### Commands Run
+- [x] Web: `npm run build:web:docker` ✅
+- [x] Docker: `docker build -t test-image .` ✅
+
+### Artifacts
+- [x] Names + **sha256** of artifacts/installers:
+
+Artifacts:
+```text
+test-image.tar  a1b2c3d4e5f6...
+```
+
+### Docs
+- [x] **BUILDING.md** updated (sections): Docker deployment
+- [x] Troubleshooting updated: Added Docker troubleshooting section
+
+### Rollback
+- [x] Verified steps to restore previous behavior:
+  1. `git revert HEAD`
+  2. `docker rmi test-image`
+  3. Restore previous BUILDING.md
+```
+
+---
+
+**Note**: This template is enforced by the Build Architecture Guard
+system. Complete all required fields to ensure your PR can be merged.