feat(gitea): implement gitea_coder role with scope enforcement (#20)

## Summary

Implements the gitea_coder role as defined in issue #11, providing a complete workflow automation layer for Git operations with scope enforcement.

## Features

### Branch Management with Scope Gating
- ✅ Enforces branch naming conventions (feature/, fix/, refactor/, docs/, test/, chore/)
- ✅ Prevents direct pushes to protected branches (main, master, develop, dev)
- ✅ Auto-appends issue numbers to branch names

### Unified Commit Workflow
- ✅ Automatic create vs replace detection
- ✅ Conventional commits format with issue references
- ✅ Detailed commit message generation

### PR Creation
- ✅ Validates source branch is not protected
- ✅ Auto-references issues in PR description
- ✅ Uses existing gitea/dev.py operations

### Ticket Integration
- ✅ Reads and parses issue requirements
- ✅ Extracts testing criteria and technical notes
- ✅ Suggests branch names from issue content

## Files Added
- `gitea/coder.py` - Complete gitea_coder role implementation

## Files Modified
- `README.md` - Added gitea_coder documentation

## Testing Criteria
✅ Can create feature branch from ticket
✅ Can modify files according to ticket requirements
✅ Can generate commit messages with issue references
✅ Can create PR for review

Refs: #11
Reviewed-on: #20

README.md

@@ -8,6 +8,7 @@ This monorepo contains a collection of automation tools for Open WebUI, designed
 Python scripts for Git operations and repository management:
 - **`admin.py`**: Administrative utilities for managing Gitea repositories, potentially including user management, permissions, and batch operations.
 - **`dev.py`**: Development-focused tools for Git workflows, branch handling, and repository interactions tailored for software development processes.
+- **`coder.py`**: Development workflow role that reads tickets, creates branches with scope enforcement, generates commit messages with issue references, and creates pull requests.
 
 ### venice/
 Tools for interacting with Venice AI services: