fix(statics): keep Singapore excluded from RBTC custody

[claude]

<!-- ccr-slack-attribution -->
Requested by Shubham Damkondwar, Kushagra Sharma · Slack thread

Description

Companion PR: BitGo/bitgo-microservices#61593 (CECHO-1849) enables RBTC custody wallets at the wallet-platform method level. Once that PR merges, any CoinFeature currently attached to RBTC_FEATURES would become eligible for RBTC custody, including Singapore.

Per the decision in the linked Slack thread ("yes we can keep it gated for singapore") and the precedent set in the old ticket COIN-5477 (gating RBTC for Singapore), Singapore trusts should remain excluded from RBTC custody. This PR makes that gating actually effective in @bitgo/statics.

Before

RBTC_FEATURES explicitly listed CoinFeature.CUSTODY_BITGO_SINGAPORE:

export const RBTC_FEATURES = [
  ...ETH_FEATURES,
  CoinFeature.MULTISIG_COLD,
  CoinFeature.MULTISIG,
  CoinFeature.CUSTODY_BITGO_GERMANY,
  CoinFeature.CUSTODY_BITGO_FRANKFURT,
  CoinFeature.CUSTODY_BITGO_SINGAPORE,
];

Simply deleting that line would not have achieved the goal: ETH_FEATURES spreads ACCOUNT_COIN_DEFAULT_FEATURES, which already includes CoinFeature.CUSTODY_BITGO_SINGAPORE by default. Since RBTC_FEATURES spreads ETH_FEATURES, the explicit entry was actually redundant with an inherited default — removing only that line would have been a no-op, and Singapore would still end up in the final RBTC_FEATURES array (and therefore still gain RBTC custody access once the companion PR ships).

After

RBTC_FEATURES now explicitly filters CUSTODY_BITGO_SINGAPORE out of the final array, the same exclusion pattern already used elsewhere in this file for coins that need to remove a feature inherited via a spread (e.g. CELO_FEATURES, ETH_FEATURES_WITH_FRANKFURT_EXCLUDE_SINGAPORE, TEMPO_FEATURES_EXCLUDE_SINGAPORE):

export const RBTC_FEATURES = [
  ...ETH_FEATURES,
  CoinFeature.MULTISIG_COLD,
  CoinFeature.MULTISIG,
  CoinFeature.CUSTODY_BITGO_GERMANY,
  CoinFeature.CUSTODY_BITGO_FRANKFURT,
].filter((feature) => feature !== CoinFeature.CUSTODY_BITGO_SINGAPORE);

This guarantees CoinFeature.CUSTODY_BITGO_SINGAPORE cannot end up on rbtc/trbtc regardless of what ETH_FEATURES / ACCOUNT_COIN_DEFAULT_FEATURES include in the future.

How this pairs with the companion PR

• BitGo/bitgo-microservices#61593 enables the RBTC custody wallet method itself; RBTC custody won't actually work anywhere until that PR also merges.
• This PR ensures that once both are merged, Singapore trusts remain blocked from RBTC custody, matching the team's decision.
• Neither PR alone changes end-user-visible behavior for Singapore; the combination does.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Confirmed via code inspection that CUSTODY_BITGO_SINGAPORE is inherited into ETH_FEATURES via ACCOUNT_COIN_DEFAULT_FEATURES, so a plain line removal would have been a no-op.
• Type-checked modules/statics/src/coinFeatures.ts in isolation (tsc --noEmit) — no errors.
• Verified RBTC_FEATURES is consumed directly by the rbtc account definition and via AccountCoin.getFeaturesByTypeExcluding for trbtc in modules/statics/src/allCoinsAndTokens.ts, so this fix covers both.

Ticket: CECHO-1849

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