feat: add Qwen Code support to Spec Kit

Add comprehensive Qwen Code integration following existing patterns:

- Add Qwen as fourth AI assistant option in CLI
- Update all documentation to include Qwen CLI references
- Extend GitHub Actions workflow for Qwen template generation
- Add Qwen support to shell scripts and templates
- Maintain backward compatibility with existing assistants

templates/plan-template.md

@@ -16,7 +16,7 @@
    → Update Progress Tracking: Initial Constitution Check
 4. Execute Phase 0 → research.md
    → If NEEDS CLARIFICATION remain: ERROR "Resolve unknowns"
-5. Execute Phase 1 → contracts, data-model.md, quickstart.md, agent-specific template file (e.g., `CLAUDE.md` for Claude Code, `.github/copilot-instructions.md` for GitHub Copilot, or `GEMINI.md` for Gemini CLI).
+5. Execute Phase 1 → contracts, data-model.md, quickstart.md, agent-specific template file (e.g., `CLAUDE.md` for Claude Code, `.github/copilot-instructions.md` for GitHub Copilot, `GEMINI.md` for Gemini CLI, or `QWEN.md` for Qwen Code).
 6. Re-evaluate Constitution Check section
    → If new violations: Refactor design, return to Phase 1
    → Update Progress Tracking: Post-Design Constitution Check
@@ -171,7 +171,7 @@ ios/ or android/
    - Quickstart test = story validation steps
 
 5. **Update agent file incrementally** (O(1) operation):
-   - Run `/scripts/update-agent-context.sh [claude|gemini|copilot]` for your AI assistant
+   - Run `/scripts/update-agent-context.sh [claude|gemini|copilot|qwen]` for your AI assistant
    - If exists: Add only NEW tech from current plan
    - Preserve manual additions between markers
    - Update recent changes (keep last 3)