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

[alirezarezvani]

Summary

Add comprehensive Phase 4 work plan for implementing GitHub App authentication support.

Changes

Added docs/PHASE4_WORKPLAN.md (1,457 lines) containing:

Objective

• Make /install-github-app the primary recommended authentication method
• Reduce setup time from 8-10 minutes to 2-3 minutes (GitHub App path)
• Keep API Key as fallback option
• Properly document OAuth Token method

7 Work Packages

WP4.1: Documentation Updates (6 files, 3-4 hours)

• Update GITHUB_SETUP.md, QUICK_START.md, COMPLETE_SETUP.md
• Update README.md, ARCHITECTURE.md, TROUBLESHOOTING.md
• Add authentication comparison tables and decision trees

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()
• Automatic fallback logic when preferred method fails

WP4.3: Update Validation Script (45 min)

• Detect which authentication method is configured
• Validate the chosen method works

WP4.4: Command & Agent Updates (1 hour)

• Update /blueprint-init command documentation
• 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 existing API Key method
• Validate fallback scenarios

WP4.7: Documentation Review & Polish (30 min)

• Proofread all updated documentation
• Verify links and consistency

Why This Matters

The blueprint currently misses the official recommended method from Claude Code Action docs. Users are forced through manual API key setup (8-10 min) when a one-command GitHub App solution exists (2-3 min).

Impact:

• ❌ Poor user experience (slow, error-prone setup)
• ❌ Doesn't align with official "best practice"
• ❌ Higher setup failure rate due to manual steps

After Phase 4:

• ✅ GitHub App as primary/default method
• ✅ 60-70% faster setup time
• ✅ Aligns with official Claude Code recommendations
• ✅ Better UX with automatic secret configuration

Deliverables

Total: 16 files to modify

• 6 documentation files
• 2 setup scripts (wizard.sh, validate.sh)
• 2 commands/agents
• 6 config templates

Estimated Time: 8-10 hours
Priority: HIGH

Testing Plan

All 3 authentication methods will be tested:

1. GitHub App flow (new)
2. API Key flow (regression test)
3. OAuth Token flow (new)

Plus fallback scenarios and documentation review.

Related

Closes #[issue-number-if-exists]

This work plan follows the same format as existing Phase 1, 2, and 3 work plans.