Lazy auto-generate + bind v3 payout wallet on first API call

[epanonymous]

Summary

• Moves payout wallet setup out of registration and into the first authenticated SDK API call.
• Adds a lazy payout-wallet binder that checks existing binding, generates a local v3 ed25519 wallet when needed, stores the secret locally, and sends only the base58 pubkey to the API.
• Serializes concurrent first calls in-process, treats server already_bound as success, and caches the bound state per API key.
• Supports caller-supplied external/passkey wallets via payoutWallet.pubkey, payoutWallet.publicKey, or payout_pubkey, plus configurable payoutWalletStorePath and payoutWalletApiBase.

Supersedes #9

This supersedes PR #9's registration-time / opt-in payout wallet binding model. PR #9 should be closed in favor of this lazy, automatic first-call binding flow.

Shared Contract

• GET /api/v1/agent/payout-wallet with agent-key auth returns { payout_pubkey: base58|null, bound_at: iso|null }.
• POST /api/v1/agent/payout-wallet/bind with agent-key auth and body { payout_pubkey } returns { payout_pubkey, bound_at, already_bound }.
• The SDK relies on the server's first-write-wins behavior for cross-process races.

Local Secret Storage

• Generated secrets are stored locally at ~/.deva/payout-wallet.json by default; callers can override this with payoutWalletStorePath.
• Store entries are keyed by an API-base/API-key fingerprint, not the raw API key; the parent directory is created with 0700 and the store file with 0600 where supported.
• The secret key is never transmitted to the server. Bind requests send only { payout_pubkey }.

Verification

• npm test (46 passing tests; HTTP calls mocked, no live API)
• npm run build
• CJS import smoke test for the lazy flow: cjs lazy payout smoke ok: status=1 bind=1 resources=2