Improve help text, refactor command registration, and fix test assertions

[ryan-williams]

This PR includes three improvements to the ghpr codebase:

1. Better Help Text and Validation

• Draft flag validation - Added error when using -d without -y, since draft mode is incompatible with web editor
• Clearer help text - Updated create command and flag descriptions to clearly explain the three modes:
  • Default (no -y): Interactive web editor (waits for user)
  • -y once: Create via API then view result
  • -yy twice: Create silently
• Auto gist push - create now automatically pushes to gist remote after finalizing PR/Issue

2. Decorator Syntax Refactoring

Converted all 10 command register() functions from deeply nested function calls to clean vertical decorator stacks:

# Before
cli.command()(
    flag('-g', '--gist')(
        flag('-n', '--dry-run')(
            command
        )
    )
)

# After
@cli.command()
@flag('-n', '--dry-run')
@flag('-g', '--gist')
def command_cmd(dry_run, gist):
    command(gist, dry_run)

This makes the code much more readable and maintainable. Net result: -32 lines of code!

3. Comprehensive Test Assertion Improvements

Fixed all test assertions across the test suite to use precise equality checks instead of fragile substring matching:

• test_import.py - Used regex to extract exact command names from CLI help, fixed shell integration checks
• test_create.py - Changed to exact tuple comparisons for gh command arguments, added missing mocks for push() call
• test_files.py - Replaced all substring checks with full content equality, fixed newline handling
• test_gist.py - Replaced substring checks with exact equality, fixed gist footer formatting expectations

Impact: Fixed 51+ problematic assertions that used assert x in y patterns. All 92 tests now pass with precise, maintainable assertions that clearly document expected values