Add Local code review with git diff and MCP server for codespy, update Readme

[pranavsriram8]

Summary of changes added in the PR

• Local Diff Builder (src/codespy/tools/git/local_diff.py)

  • Builds MergeRequest from git diff
  • Supports committed changes (branch vs base)
  • Supports uncommitted changes (working tree)

• MCP Server (src/codespy/agents/reviewer/server.py)

  • 3 tools: review_local_changes, review_uncommitted, review_pr

• CLI Commands (src/codespy/cli.py)

  • codespy review-local - Review branch vs base
  • codespy review-uncommitted - Review working tree changes
  • codespy serve - Start MCP server

• Complete Documentation (README.md)

  • Usage examples for all modes
  • Environment variable setup for CLI/MCP