fix: rewrite skill.md with accurate per-scanner method support matrix

- Replace vague all-methods-everywhere claim with explicit support matrix
- blockscout_v2 supports ONLY 6 methods (balance, transactions, token
  portfolio, contract ABI, ENS reverse lookup x2)
- Add scanner selection guide at top (choose scanner by task)
- Document tx schema differences: blockscout_v2 uses nested from/to dicts
- Add accurate network caveats (polygon 500, optimism 301 redirect)
- Add per-scanner quick start examples and error handling patterns
- Validated by real test agent: 5/7 tasks succeeded, 2 failed expectedly
  (get_block/gas_oracle on mainnet without etherscan key, as documented)

Author: VaitaR