feat(claw402): preflight USDC balance before AI calls (#1479)

* chore: ignore nofx-server build artifact

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat(claw402): preflight USDC balance before AI calls

Short-circuit claw402 Call/CallWithRequestFull when the wallet balance
can't cover the estimated cost of the call, surfacing ErrInsufficientFunds
instead of letting x402 fail mid-flight after the sign step.

- wallet: cached balance lookup (30s TTL, per-address mutex) to avoid
  hammering the Base RPC; separate error-returning and display-only APIs
  so callers can distinguish zero balance from an unreachable RPC.
- claw402: 1.5× safety multiplier on the flat per-call estimate, 4.0×
  for reasoner models whose chain-of-thought cost can blow past the
  flat rate. Fail-open on RPC errors — x402 still gates actually-empty
  wallets, and we prefer availability over extra strictness.
- shortAddr redacts the wallet in error strings to avoid leaking the
  full address into telemetry bundles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -126,3 +126,6 @@ dmypy.json
 # Pyre type checker
 .pyre/
 PR_DESCRIPTION.md
+
+# Go build artifacts
+/nofx-server