Improve CLI help formatting for command groups

[dafeda]

Render command-group help with standard usage text and one command per line, and section leaf subcommand help into usage, options, returns, and examples.

Summary

Before fix:

➜  nansen-cli git:(main) nansen research portfolio defi --help
portfolio defi — DeFi holdings across protocols
Params (* required): --wallet*
Cost: 10 credits (Free tier) / 1 credit (Pro tier)
Example: nansen research portfolio defi --wallet <val>

After fix:

➜  nansen-cli git:(cli-help) nansen research portfolio defi --help
USAGE: nansen research portfolio defi [options]

DeFi holdings across protocols

OPTIONS (* required):
  --wallet*

Cost: 10 credits (Free tier) / 1 credit (Pro tier)

EXAMPLE:
  nansen research portfolio defi --wallet <val>

Before fix:

➜  nansen-cli git:(main) nansen research --help
research — Research and analytics commands
Subcommands: portfolio, smart-money, profiler, token, search, perp, prediction-market, points, historical-dex-trades, historical-pnl-leaderboard, historical-token-flow-summary, historical-token-quant-scores, historical-top-holders, historical-who-bought-sold, historical-smart-money-balances, historical-token-screener, historical-wallet-balances, historical-tx-lookup, historical-wallet-transactions
Use: nansen research <subcommand> --help

After fix:

➜  nansen-cli git:(cli-help) nansen research --help
USAGE: nansen research <command> [options]

Research and analytics commands

COMMANDS:
  portfolio
  smart-money
  profiler
  token
  search
  perp
  prediction-market
  points
  historical-dex-trades
  historical-pnl-leaderboard
  historical-token-flow-summary
  historical-token-quant-scores
  historical-top-holders
  historical-who-bought-sold
  historical-smart-money-balances
  historical-token-screener
  historical-wallet-balances
  historical-tx-lookup
  historical-wallet-transactions

Run 'nansen research <command> --help' for more information on a command.

Checklist

• Tests pass (npm test)
• src/schema.json updated if new commands or flags were added
• README.md updated if new top-level commands or categories were added
• Changeset added (.changeset/) — only required for user-facing changes (new commands, flags, bug fixes, breaking changes)