feat: add batch-report CLI command to generate Markdown or HTML reports from existing batch runs

[claytonlin1110]

Summary

Adds a batch-report subcommand that turns an existing batch run’s batch_report.json into a Markdown or HTML report.
Closes #84

Changes

• paperbanana/core/batch.py: load_batch_report(), generate_batch_report_md(), generate_batch_report_html(), write_batch_report().
• paperbanana/cli.py: New paperbanana batch-report command with --batch-dir / --batch-id, --output, and --format (markdown | html).
• tests/test_batch.py: Tests for load/generate/write and invalid inputs.
• README.md: Documented batch-report usage in the batch section.

Usage

# By path
paperbanana batch-report --batch-dir outputs/batch_20250109_123456_abc --format markdown

# By batch ID (under default output dir)
paperbanana batch-report --batch-id batch_20250109_123456_abc -f html -o report.html

[claytonlin1110]

@dippatel1994 Please review

[dippatel1994]

Looks good, thanks @claytonlin1110 for your contribution!