feat(skills): merge blockchain/base into blockchain/evm; salvage PR #2010

Salvages the closed PR #2010 (Mibayy's EVM multi-chain skill) and folds the
existing optional-skills/blockchain/base/ skill into it, so we ship one
unified EVM skill instead of two overlapping ones.

Pulled in from base/:
  - 8 missing Base-specific tokens (AERO, DEGEN, TOSHI, BRETT, WELL,
    cbETH, cbBTC, wstETH, rETH) added to KNOWN_TOKENS['base'] —
    base/ had 11, evm/ only had 3 (USDC/DAI/WETH).
  - L1 data-fee pitfall note for rollups (Base, Arbitrum, Optimism, zkSync).
  - Batch-size chunking in rpc_batch (Base RPC caps batches at 10 calls
    per JSON-RPC request; adding more known tokens tripped that limit
    and broke 'wallet --chain base' with a 'list index out of range'
    error). Ported the chunking pattern from base/_rpc_batch_chunk.

Latent bugs found and fixed while smoke-testing the merge:
  - cmd_multichain and cmd_allowance both iterated KNOWN_TOKENS[chain]
    with 'for contract, (symbol, _name) in known.items()' — but the dict
    shape is {symbol: contract_str}, not {addr: (sym, name)}. This raised
    'too many values to unpack (expected 2)' on every non-zero balance.
    Now iterates as 'for symbol, contract in known.items()'.
  - Input validation: added is_valid_address / is_valid_txhash /
    require_address / require_txhash helpers and wired them into
    cmd_wallet, cmd_tx, cmd_token, cmd_activity, cmd_allowance,
    cmd_decode, cmd_contract, cmd_multichain. Fails fast with exit 2
    on malformed input instead of burning an RPC round-trip on garbage.

Documentation:
  - SKILL.md now flags that this skill supersedes optional-skills/blockchain/base.
  - Pitfalls expanded for ENS (single-endpoint dependency on
    ensideas.com), tx decoding (single-endpoint dependency on
    4byte.directory), and rollup L1 fees.
  - Regenerated website/docs/user-guide/skills/optional/blockchain/
    blockchain-evm.md and removed the old blockchain-base.md page;
    catalog updated.

Removed:
  - optional-skills/blockchain/base/SKILL.md
  - optional-skills/blockchain/base/scripts/base_client.py
  - website/docs/user-guide/skills/optional/blockchain/blockchain-base.md

Smoke-tested live against Base mainnet: stats, price, token, wallet
(vitalik.eth — 3.12 ETH + 13.88 USDC + 4.23 DAI + 0.06 WETH on Base)
and allowance (ethereum, 7 unlimited approvals to Uniswap/Permit2).

Original PR #2010 author: Mibayy.
Original base/ skill author: youssefea.

optional-skills/blockchain/evm/SKILL.md

@@ -25,6 +25,11 @@ Optimism, Avalanche (C-Chain), zkSync Era.
 No API key needed. Zero external dependencies — Python standard library only
 (urllib, json, argparse, threading).
 
+> **Supersedes the standalone `base` skill.** Base-specific tokens (AERO, DEGEN,
+> TOSHI, BRETT, WELL, cbETH, cbBTC, wstETH, rETH) and all Base RPC functionality
+> previously living under `optional-skills/blockchain/base/` have been folded
+> into this skill. Pass `--chain base` to any command for Base coverage.
+
 ---
 
 ## When to Use
@@ -188,8 +193,10 @@ Shows gwei price + USD cost for: transfer, ERC-20 transfer, approve, swap, NFT m
 - `wallet` and `allowance` only check known token list (~30 tokens per chain). Use a block explorer for complete token discovery.
 - `activity` scans recent blocks only (max 200). For full history, use Etherscan API.
 - `multichain` runs 8 parallel threads — can trigger rate limits on public RPCs.
-- ENS requires internet access to ensideas.com.
-- Tx decode requires internet access to 4byte.directory.
+- ENS resolution depends on a single public endpoint (ensideas.com / ens.vitalik.ca) with no fallback. If that endpoint is down, `ens` will fail — re-run later or use a block explorer.
+- Tx decoding depends on a single public endpoint (4byte.directory) with no fallback. Selectors not in their database show up as `unknown`.
+- **L2 gas estimates are L2-execution only.** On rollups like Base, Arbitrum, Optimism, and zkSync, the actual transaction cost also includes an L1 data-posting fee that depends on calldata size and current L1 gas prices. The `gas` command does not estimate that L1 component. For Base specifically, see the network's L1 fee oracle (contract `0x420000000000000000000000000000000000000F`).
+- Address / tx-hash inputs are validated for 0x-prefix + correct length + hex, but EIP-55 checksum casing is **not** enforced (RPC endpoints accept any-case hex).
 
 ---