feat: auto-merge non-breaking dependency updates (#5)

## Summary
Minori now automatically merges dependency update PRs when they meet safety criteria, reducing manual work whilst maintaining safety for potentially breaking changes.

## Changes
- ✨ Add version comparison utility to detect major version bumps
- ✨ Add Gitea service methods for checking commit status and merging PRs
- ✨ Add auto-merge logic that checks:
  - Is it a major version bump? (if yes, skip auto-merge)
  - Did CI checks pass? (if no, skip auto-merge)
  - If both conditions pass → auto-merge! 🎉
- ✅ Add comprehensive tests for all new functionality
- 📊 Maintain ~94% test coverage

## How It Works
When Minori processes a dependency update:
1. Check if a PR already exists for that dependency
2. If it exists, verify:
   - **Not a major version bump** (major bumps need manual review)
   - **CI status = "success"** (all checks must pass)
3. If both conditions are met → automatically merge the PR and delete the branch

## Test Plan
- [x] All 114 tests passing
- [x] New tests for version comparison utility
- [x] New tests for Gitea service extensions
- [x] Build successful
- [x] Linting clean

---
✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Reviewed-on: #5
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

package.json

@@ -20,11 +20,11 @@
     "@types/node": "25.2.0",
     "@types/node-cron": "3.0.11",
     "@types/semver": "7.7.1",
-    "@vitest/coverage-istanbul": "^4.0.18",
-    "@vitest/coverage-v8": "^4.0.18",
+    "@vitest/coverage-istanbul": "4.0.18",
+    "@vitest/coverage-v8": "4.0.18",
     "eslint": "9.39.2",
     "typescript": "5.9.3",
-    "vitest": "^4.0.18"
+    "vitest": "4.0.18"
   },
   "dependencies": {
     "@nhcarrigan/logger": "1.1.1",