fix(shotcut): add auto-save option for one-shot mode

[Jah-yee]

Summary

Add -s/--save flag to automatically save project after each mutation command. This fixes the issue where one-shot commands don't persist changes because each command runs in a new process with a fresh session.

Changes

• Add auto_save global flag to CLI group
• Add -s/--save option to the main CLI
• Implement _auto_save_callback() that runs after each command
• Auto-save only in one-shot mode (not REPL mode)

Usage

# Before (changes not persisted):
shotcut-cli --project edit.mlt timeline add-track --type video
shotcut-cli --project edit.mlt project info  # track_count: 0

# After (with -s flag):
shotcut-cli --project edit.mlt -s timeline add-track --type video
# Auto-saved to: edit.mlt
shotcut-cli --project edit.mlt project info  # track_count: 1

Test

Verified the fix works:

• Created new project
• Added track with -s flag
• Track persisted in project file

Fixes #14

[jarrodcolburn]

If the repl subcommand isn't used it might be assumed that that changes will be saved, in which case --save may be redundant requirement to need to save when running commands.

[yuh-yang]

Nice catch! Can we improve HARNESS.md to solve this fundamentally? I'll merge this first.

[Jah-yee]

If the repl subcommand isn't used it might be assumed that that changes will be saved, in which case --save may be redundant requirement to need to save when running commands.

@jarrodcolburn
Thanks for the thoughtful feedback on one-shot expectations!
I followed up with a docs PR to clarify the Shotcut CLI execution & persistence model in HARNESS.md and to document how -s/--save behaves in one-shot vs REPL: #29.

This keeps current behavior but makes the design and trade-offs explicit, and also notes the potential future direction of defaulting to auto-save for one-shot with a --no-save opt-out if that better matches real-world usage.