docs: add Phase 4 work plan for GitHub App authentication support

Added comprehensive Phase 4 work plan (PHASE4_WORKPLAN.md) covering:

**Objective**: Add GitHub App authentication as primary method
- Reduce setup time from 8-10 min to 2-3 min
- Make /install-github-app the recommended approach
- Keep API Key as fallback option
- Document OAuth Token properly

**7 Work Packages** (8-10 hours total):
- WP4.1: Documentation Updates (6 files, 3-4 hours)
  - GITHUB_SETUP.md, QUICK_START.md, COMPLETE_SETUP.md
  - README.md, ARCHITECTURE.md, TROUBLESHOOTING.md
  - Add comparison tables and authentication architecture

- WP4.2: Setup Wizard Enhancement (2-3 hours)
  - Interactive authentication menu (GitHub App, API Key, OAuth)
  - 3 new functions: setup_github_app(), setup_api_key(), setup_oauth_token()
  - Fallback logic when preferred method fails

- WP4.3: Update Validation Script (45 min)
  - Detect authentication method
  - Validate chosen method works

- WP4.4: Command & Agent Updates (1 hour)
  - Update /blueprint-init command
  - Update blueprint-setup agent

- WP4.5: Configuration Template Updates (30 min)
  - Add auth method field to all 6 config files

- WP4.6: Testing & Validation (2 hours)
  - Test all 3 authentication flows end-to-end
  - Regression testing for API Key method

- WP4.7: Documentation Review & Polish (30 min)
  - Proofread all docs
  - Verify links and consistency

**Total Deliverables**: 16 files to modify
**Format**: Matches PHASE1/2/3 work plan structure
**Priority**: HIGH - comprehensive implementation

This addresses the critical gap where the blueprint was missing
the official recommended GitHub App installation method, forcing
users through slower manual API key setup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: alirezarezvani