[Core] Implement isomorphic-git service for mobile Git operations

[coderabbitai]

Overview

Build the core Git service using isomorphic-git for mobile-native repository management.

Current State

• isomorphic-git dependency installed but completely unused
• Types defined in src/types/index.ts (Repository, Workspace, FileChange, DiffHunk)
• No implementation exists

Requirements

• Create src/services/git/ directory structure
• Implement GitService class with methods:
  • clone(url, credentials)
  • fetch/pull/push
  • commit/stage/unstage
  • branch operations (create, checkout, list)
  • diff generation
  • status checking
• Integrate with React Native FileSystem
• Add progress callbacks for long operations
• Implement error handling and retry logic
• Write comprehensive unit tests
• Create usage documentation

Success Criteria

• Can clone, commit, and push to GitHub from mobile
• Full diff viewing capability
• Handles auth with tokens from SecureStore

• 85% test coverage

Priority: CRITICAL

Estimated Effort: 5-7 days

Dependencies: Credential Service (Issue TBD)

Labels: core, git, priority:critical

Backlink: Related to PR #2