docs: wallet layer refactor brainstorm

[its-everdred]

Summary

Brainstorm doc for a wallet layer refactor: align with the lend/borrow LendProvider<TConfig> pattern, rename for accuracy (EOAWallet → ExternalWallet, DefaultSmartWallet → CoinbaseSmartWallet), collapse duplicate node/react code, and make room for ZeroDevSmartWallet (needed for agent session keys).

• Research only — no code changes
• Sub-issues will be opened separately and linked to a parent tracking issue
• Preserves ActionsConfig shape; preserves node/react split; reduces ~35 wallet files to ~20

Related

• Supersedes Agent wallet support research #354 on the hosted→embedded question
• Folds in Wallet Refactor: Rename hosted wallet provider to embedded wallet provider #330 (rename) and Wallet Refactor: Rename send/sendTokens (viem alignment) and DRY up token transfer + signing #370 (send/sendTokens + signing hoist) as sub-issues
• Ships after Wallet Refactor: Support local EOAs and make embedded optional #356 (local wallet provider) merges
• Considered Metamask standard Research: MetaMask Advanced Permissions (ERC-7715/7710) #358

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