feat(cli): add progress bars to tool pack and improve UX

[appcypher]

Summary

• Add real-time progress indicators for tool pack with visual feedback during bundle creation
• Show scrolling file names while packing single bundles, parallel progress bars for multi-platform packing
• Add validation warning W018 for script names that conflict with built-in subcommands
• Improve Windows compatibility for shell command execution
• Update README with expanded feature list

This improves the developer experience by providing visual feedback during potentially long-running pack operations, making it clear what files are being processed and how far along the operation is.

Changes

Progress System (lib/pack.rs, lib/handlers/tool/pack_cmd.rs):

• Add PackProgress enum with Started, FileAdded, and Finished variants
• Add ProgressCallback type and on_progress field to PackOptions
• Refactor pack_bundle and pack_bundle_for_platform to emit progress events
• Add extract_icon option to control icon file creation (pack vs publish)

Pack Command UI (lib/handlers/tool/pack_cmd.rs):

• Single bundle mode: progress bar with 3 scrolling file names below
• Multi-platform mode: parallel progress bars per platform (no file names)
• Add validation spinner for strict mode

Script Name Validation (lib/validate/validators/scripts.rs - NEW):

• Add W018 warning code for reserved script names
• Validate script names don't conflict with built-in subcommands (init, install, run, etc.)
• Integrate into manifest validation pipeline

Windows Compatibility (lib/handlers/tool/detect_cmd.rs, lib/handlers/tool/scripts.rs):

• Use cmd /C on Windows instead of sh -c for shell execution

Documentation (README.md):

• Add expanded feature list with icons

Test Plan

• Run cargo build to verify compilation
• Run cargo test --lib pack:: to verify pack tests pass
• Test single bundle packing: tool pack <path> - should show progress bar with scrolling files
• Test multi-platform packing: tool pack --multi-platform <path> - should show parallel progress bars
• Test strict validation: tool pack --strict <path> - should show validation spinner
• Verify no icon files are created by tool pack (only tool publish should create them)