fix: use only staged files by default, add --all flag

[vavasilva]

Summary

• Fixes Comportamento inesperado: CLI adiciona todos os arquivos ao invés de usar apenas os staged #13: CLI now respects pre-staged files instead of staging everything
• Adds -a, --all flag for users who want to stage all changes before committing
• Updates README with new usage examples and documentation

Changes

• src/cli.ts: Changed default behavior to only use staged files; added --all flag
• README.md: Updated examples and CLI options table

New Behavior

Command	Behavior
git add file.ts && git-commit-ai	Commits only staged files
git-commit-ai --all	Stages everything first (previous default behavior)

Test plan

• npm run typecheck passes
• npm run test passes (61 tests)
• npm run build succeeds
• --help shows new -a, --all flag
• Manual test: stage one file, run git-commit-ai, verify only that file is committed
• Manual test: run git-commit-ai --all, verify all changes are staged and committed

🤖 Generated with Claude Code