feat(#156): Create coordinator bot user documentation and setup scripts

Add comprehensive documentation and automated scripts for setting up the mosaic
coordinator bot user in Gitea. This enables the coordinator system to manage
issue assignments, comments, and orchestration.

Changes:
- docs/1-getting-started/3-configuration/4-gitea-coordinator.md: Complete setup guide
  * Step-by-step bot user creation via UI and API
  * Repository permission configuration
  * API token generation and storage
  * Comprehensive testing procedures
  * Security best practices and troubleshooting

- scripts/coordinator/create-gitea-bot.sh: Automated bot creation script
  * Creates mosaic bot user with proper configuration
  * Sets up repository permissions
  * Generates API token
  * Tests authentication
  * Provides credential output for secure storage

- scripts/coordinator/test-gitea-bot.sh: Bot functionality test suite
  * Tests authentication
  * Verifies repository access
  * Tests issue operations (read, list, assign, comment)
  * Validates label management
  * Confirms all required permissions

- scripts/coordinator/README.md: Scripts usage documentation
  * Workflow guides
  * Configuration reference
  * Troubleshooting section
  * Token rotation procedures

- .env.example: Added Gitea coordinator configuration template
  * GITEA_URL, GITEA_BOT_USERNAME, GITEA_BOT_TOKEN
  * GITEA_BOT_PASSWORD, GITEA_REPO_OWNER, GITEA_REPO_NAME
  * Security notes for credential storage

All acceptance criteria met:
✓ Documentation for bot user creation
✓ Automated setup script
✓ Testing procedures and scripts
✓ Configuration templates
✓ Security best practices
✓ Troubleshooting guide

Addresses Milestone: M4.1-Coordinator
Relates to: #140, #157, #158

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.env.example

@@ -142,6 +142,22 @@ TRAEFIK_ACME_EMAIL=admin@example.com
 TRAEFIK_DASHBOARD_ENABLED=true
 TRAEFIK_DASHBOARD_PORT=8080
 
+# ======================
+# Gitea Integration (Coordinator)
+# ======================
+# Gitea instance URL
+GITEA_URL=https://git.mosaicstack.dev
+
+# Coordinator bot credentials (see docs/1-getting-started/3-configuration/4-gitea-coordinator.md)
+# SECURITY: Store GITEA_BOT_TOKEN in secrets vault, not in version control
+GITEA_BOT_USERNAME=mosaic
+GITEA_BOT_TOKEN=REPLACE_WITH_COORDINATOR_BOT_API_TOKEN
+GITEA_BOT_PASSWORD=REPLACE_WITH_COORDINATOR_BOT_PASSWORD
+
+# Repository configuration
+GITEA_REPO_OWNER=mosaic
+GITEA_REPO_NAME=stack
+
 # ======================
 # Logging & Debugging
 # ======================