feat(cli): add leaderboard subcommand with markdown output

[hummbl-dev]

Summary

• Adds leaderboard subcommand that reads from an existing DB and outputs quality rankings as a formatted markdown table
• Adds --markdown / --md flag to github-top for inline markdown output after scoring
• Supports --output flag to write markdown to file, --lang for header label
• 15 new tests covering markdown formatting, CLI integration, file output, and help

Test plan

• Markdown output contains expected headers, table markers, and footer
• Repo links rendered for org/repo slugs, plain text for bare names
• --output writes to file correctly
• leaderboard --help shows expected flags
• Missing DB exits with error
• --markdown flag registered on github-top
• Full test suite (238 tests) passes

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Accept --db on leaderboard parser

The new command is documented as python -m arbiter leaderboard [--db FILE], but leaderboard itself does not register a --db option, so python -m arbiter leaderboard --db /path/to.db exits with unrecognized arguments. Because this subcommand is new, users are likely to pass flags after the subcommand and will hit a hard failure instead of being able to target an existing database.

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