Added comprehensive documentation for the --prefix flag that enables
monorepo support with tag prefixes like pkg/name/vX.X.X. This feature
was already implemented in the codebase but was not documented.

Changes:
- Updated README.md with monorepo support section and examples
- Added --json and --prefix flags to options documentation
- Added test cases for prefix functionality in git_test.go
- Documented expected behavior for various monorepo scenarios

All tests pass successfully.

Refactored the codebase to be cleaner, more maintainable, and more
idiomatic Go without changing functionality.

Changes in internal/cmd/bump.go:
- Extracted runBump function from inline cobra.Command handler
- Added helper functions: getCurrentVersion, normalizePrefix, formatTag, printVersionChange
- Simplified gitStateChecks with helper functions: handleGitError, handleCheckFailure, checkCondition, handleRemoteTags
- Reduced code duplication and improved readability
- Better error handling with clearer flow

Changes in internal/cmd/run.go:
- Split Run() into focused functions: createOptions, setupCommands, printStartupMessages, outputJSON
- Removed duplicated JSON output logic
- Simplified helper functions with cleaner implementations
- Removed overly verbose comments
- Added missing import for cobra.Command

Changes in internal/git/git.go:
- Added runGitCommand helper to reduce duplication
- Extracted getCurrentBranch with fallback logic
- Added hasRemote helper function
- Refactored all Cmd* functions to use helpers
- Simplified error handling patterns
- Improved code readability and reduced nesting
- Cleaned up comments to be more concise

All tests pass successfully. Binary builds and runs correctly.

Removed the unnecessary wrapper function in RunE. Instead of:
  RunE: func(cmd *cobra.Command, args []string) error {
      return runBump(cmd, args, opts)
  }

Now using a closure that captures opts directly:
  RunE: func(cmd *cobra.Command, args []string) error {
      // implementation here
  }

This is more idiomatic Go - using closures to capture state rather
than passing extra parameters through wrapper functions.

Benefits:
- Eliminates unnecessary function indirection
- More idiomatic Go pattern
- Cleaner code structure
- Still maintains all helper functions for testability