feat: use -a for add, --amend for amend
Changed flag behavior:
- -a is now shorthand for --add (stage all modified files)
- --amend is now long-form only (no short flag)
Updated documentation in main.go, AGENTS.md, and README.md to reflect
new flag behavior.
Closes: bug-f90a6de
Assisted-by: Claude Sonnet 4.5 via Crush