feat[mcp]: add critical missing git primitives to MCP server

## Summary
Add essential git operations that are currently missing from the git MCP server.

## Critical Missing Primitives
These are fundamental operations needed for daily git workflows:

- [ ] **git_merge** - Essential for integrating branches
- [ ] **git_rebase** - For maintaining linear history
- [ ] **git_fetch** - To update remote refs without merging
- [ ] **git_stash** / **git_stash_pop** - For temporary work storage
- [ ] **git_tag** - For version marking
- [ ] **git_cherry_pick** - For selective commit application

## Implementation Notes
- Each tool should follow the existing pattern with Pydantic models
- Include proper error handling and logging
- Integrate with git_batch for chaining operations
- Add comprehensive descriptions for each tool

## Priority
HIGH - These are blocking common workflows

Principle: developer-experience

No files, tests, or entry points are named in the issue. Start by locating the existing git MCP tool implementations and git_batch integration, then compare their Pydantic models, error handling, logging, and descriptions; done means all six listed primitives are available and support the stated chaining and documentation requirements.

Written by the indexing model from the issue text.