feat[mcp]: add git_tag tool to MCP server

## Summary
Add git_tag tool for version marking - essential for release management.

## Requirements
- Create lightweight and annotated tags
- List existing tags
- Delete tags (local and remote)
- Push tags to remote
- Support tag messages and signatures

## Implementation
```python
class GitTag(BaseModel):
repo_path: str
action: str = "create" # "create", "list", "delete", "push"
tag_name: str | None = None # For create/delete
message: str | None = None # For annotated tags
ref: str = "HEAD" # Commit to tag
annotated: bool = True # Create annotated vs lightweight
force: bool = False # Force create/update
remote: str = "origin" # For push action
```

## Priority
CRITICAL - Required for version management

Related to: #534

Principle: developer-experience

Start by locating the MCP server's tool-registration entry point and compare the related work in issue #534. Implement the GitTag actions described here, including annotated or lightweight creation, listing, deletion, pushing, messages, signatures, refs, remotes, and force handling. Done means each action works against a repository and the tool is exposed through the MCP server.

Written by the indexing model from the issue text.