feat: add cg token-price command for contract address lookups

[gosuto-inzasheru]

Summary

Adds a new token-price command that fetches current prices by token contract addresses via the /simple/token_price/{platform} endpoint.

cg token-price --address 0x1f9840a85d5af5bf1d1762f925bdaddc4201f984 --platform ethereum
cg token-price --address 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48,0xdAC17F958D2ee523a2206206994597C13D831ec7 --platform ethereum
cg token-price --address 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 --platform base --vs eur
cg token-price --address 0x912CE59144191C1204E64559FE8253a0e49E6548 --platform arbitrum-one -o json

Motivation

The existing cg price command supports coin IDs and ticker symbols, but many web3 workflows start with a contract address. Currently there's no way to look up a token's price by address without first resolving it to a CoinGecko ID. This command fills that gap using the existing /simple/token_price/{platform} API endpoint.

Implemented as a standalone command rather than flags on cg price because:

• Different API endpoint (/simple/token_price/{platform} vs /simple/price)
• Different input type (contract addresses vs coin IDs/symbols)
• Response keys are contract addresses, not coin IDs
• Avoids flag interdependency (--platform required with --contract but invalid with --ids)

Changes

• cmd/token_price.go — new command with --address, --platform, --vs flags
• cmd/token_price_test.go — 7 tests (missing flags, dry-run, JSON output, multiple addresses, no results, partial miss warning)
• internal/api/coins.go — new SimpleTokenPrice client method
• internal/api/coins_test.go — 3 API client tests (basic, multiple addresses, platform escaping)
• cmd/commands.go — command metadata for agent catalog

Features

• Multiple comma-separated addresses in one call
• --vs currency support (default: usd)
• --dry-run mode
• -o json output
• Partial miss warnings on stderr
• Registered in cg commands catalog for agent discovery