fix(commit-commands): handle initial commit when no commits exist yet

[sakumoto-shota]

Summary

• Fix /commit-commands:commit failing with fatal: your current branch does not have any commits yet

Changes

• plugins/commit-commands/commands/commit.md: Add error handling for repos with no commits
  • git diff HEAD falls back to git diff --cached (shows staged changes without HEAD)
  • git log --oneline -10 falls back to "No commits yet (initial commit)"

Testing

Tested in a fresh git init repo with no commits:

• git diff HEAD 2>/dev/null || git diff --cached — shows staged diff correctly
• git log --oneline -10 2>/dev/null || echo "No commits yet (initial commit)" — shows fallback message
• Both commands still work normally in repos with existing commits

Fixes #24881