Redesign CLI with unified format-agnostic interface

[dguido]

Summary

Replaces the nested subcommand approach with a flat, auto-detecting CLI interface:

• Auto-detection: The tool figures out if input is PyTorch, TorchScript, plain pickle, etc.
• Flat commands: No nesting, just fickling <verb> <file>
• Backward compatible: All existing flags (--check-safety, --inject, etc.) still work

New CLI Commands

# Core commands (auto-detect format)
fickling FILE                           # Decompile any pickle/model
fickling check FILE                     # Safety check any format
fickling inject FILE -c CODE -o OUT     # Inject into any format
fickling info FILE                      # Show format + properties
fickling create-polyglot F1 F2 -o OUT   # Create polyglot file

# Backward compatible (still works)
fickling --check-safety FILE            # Same as 'fickling check FILE'
fickling --inject CODE FILE             # Legacy injection syntax

Example Usage

# Just analyze a file (any format)
$ fickling model.pth
result0 = _rebuild_tensor_v2(...)

# Check if it's safe
$ fickling check model.pth
Detected format: PyTorch v1.3
No unsafe operations found.

# Get detailed info
$ fickling info model.pth
Format: PyTorch v1.3
Properties:
  is_torch_zip: true
  has_data_pkl: true
  ...

# Inject payload (works on any supported format)
$ fickling inject model.pth -c "print('pwned')" -o evil.pth
Detected format: PyTorch v1.3
Payload injected. Output: evil.pth

Key Changes

File	Change
fickling/loader.py	Added auto_load() function for format detection
fickling/cli.py	Complete rewrite with flat command structure
fickling/cli_pytorch.py	DELETED - functionality absorbed into cli.py
fickling/cli_polyglot.py	DELETED - functionality absorbed into cli.py
test/test_cli.py	Updated tests for new interface

Design Decisions

1. Auto-detection via auto_load() - Tries PyTorch ZIP formats first (most common for ML), falls back to plain pickle
2. Manual routing - Uses _get_first_positional() to detect if first arg is a command vs file
3. Separate parsers - _create_command_parser() for new commands, _create_legacy_parser() for backward compat
4. Unified handlers - Same _handle_check, _handle_inject, etc. work for any format

Test plan

• All 20 CLI tests pass
• All 41 tests in test_pickle.py and test_cli.py pass
• Backward compatibility verified (--check-safety, --inject flags work)
• Linting passes

Closes #101

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