feat: create parent directories for output files automatically

[dyoshikawa-claw]

Summary

When --output points to a path inside a directory that does not exist yet, ghactivities currently fails instead of creating the directory automatically.

Motivation

A first-run example like --output ./tmp/ghactivities.json should work without requiring the user to create ./tmp manually.

Expected behavior

• Create the parent directory for --output when it does not exist
• Keep existing behavior for plain file paths in the current directory
• Return a clear error only when directory creation is impossible

Notes

This was observed during a real local test run of the CLI.