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>

test/services/updateOrchestratorService.spec.ts

@@ -18,6 +18,8 @@ const mockGiteaGetFileContent = vi.fn();
 const mockGiteaListOrgRepositories = vi.fn();
 const mockGiteaCreatePullRequest = vi.fn();
 const mockGiteaListPullRequests = vi.fn();
+const mockGiteaGetCommitStatus = vi.fn();
+const mockGiteaMergePullRequest = vi.fn();
 const mockNpmGetPackageChangelog = vi.fn();
 const mockNpmGetPackageInfo = vi.fn();
 const mockAnalyzePackageJson = vi.fn();
@@ -39,9 +41,11 @@ vi.mock("../../src/services/giteaService.js", () => {
 
     GiteaService: class MockGiteaService {
       public createPullRequest = mockGiteaCreatePullRequest;
+      public getCommitStatus = mockGiteaGetCommitStatus;
       public getFileContent = mockGiteaGetFileContent;
       public listOrgRepositories = mockGiteaListOrgRepositories;
       public listPullRequests = mockGiteaListPullRequests;
+      public mergePullRequest = mockGiteaMergePullRequest;
     },
   };
 });
@@ -221,6 +225,7 @@ describe("updateOrchestratorService", () => {
     const mockUpdates = [ createMockUpdate() ];
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockNpmGetPackageChangelog.mockResolvedValue("## Changelog");
     mockCloneRepository.mockResolvedValue(createMockClonedRepo());
@@ -267,6 +272,7 @@ describe("updateOrchestratorService", () => {
     const mockUpdates = [ createMockUpdate() ];
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockCloneRepository.mockResolvedValue(createMockClonedRepo());
     mockCreateOrUpdateBranch.mockResolvedValue({
@@ -289,6 +295,7 @@ describe("updateOrchestratorService", () => {
     const mockUpdates = [ createMockUpdate() ];
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockCloneRepository.mockResolvedValue(createMockClonedRepo());
     mockCreateOrUpdateBranch.mockResolvedValue({
@@ -312,6 +319,7 @@ describe("updateOrchestratorService", () => {
     const mockUpdates = [ createMockUpdate() ];
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockCloneRepository.mockResolvedValue(createMockClonedRepo());
     mockCreateOrUpdateBranch.mockResolvedValue({
@@ -354,6 +362,7 @@ describe("updateOrchestratorService", () => {
     const mockUpdates = [ createMockUpdate() ];
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockCloneRepository.mockResolvedValue(createMockClonedRepo());
     mockCreateOrUpdateBranch.mockResolvedValue({
@@ -377,6 +386,7 @@ describe("updateOrchestratorService", () => {
     const mockUpdates = [ createMockUpdate() ];
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockNpmGetPackageChangelog.mockResolvedValue("## Changelog");
     mockCloneRepository.mockResolvedValue(createMockClonedRepo());
@@ -426,6 +436,7 @@ describe("updateOrchestratorService", () => {
     const mockCleanup = vi.fn();
     mockGiteaListOrgRepositories.mockResolvedValue(mockRepos);
     mockGiteaGetFileContent.mockResolvedValue(mockFileContent);
+    mockGiteaListPullRequests.mockResolvedValue([]);
     mockAnalyzePackageJson.mockResolvedValue(mockUpdates);
     mockCloneRepository.mockResolvedValue(createMockClonedRepo(mockCleanup));
     mockCreateOrUpdateBranch.mockResolvedValue({