fix: normalize selectors and ban conditional empty spreads

[K-Mistele]

Summary

• normalize blank optional subagent selector strings before enforcing the exactly-one selector invariant
• preserve dispatch, resume, and fork behavior for Grok-style empty-string / false placeholders
• vendor and enable anti-slop/no-conditional-empty-object-spread from K-Mistele/anti-slop at cf9bad836a7ba5562f5167a3471c97a1849a9f5f
• replace all 145 detected conditional empty-object spreads with typed local builders: create the stable object once, then assign optional properties only when present
• retain direct control-flow branches only for genuinely distinct semantic or discriminated variants; never use a whole-object ternary merely to evade the rule

Context

Grok 4.6 emitted agent_id: "" for inactive selectors. The prior parser counted that as active, so even intended fork calls failed before a child agent launched. Codex omits these inactive fields.

Validation

• bun run lint — 0 errors (existing warnings only)
• bun run format:check
• bun run typecheck
• bun run test
• git diff --check

Release

The selector normalization is a consumer-visible @humanlayer/fold-core bug fix. Fold uses tag-based releases (no Changesets); publish it in the next patch tag, v0.1.17, then bump Synclayer's Fold dependencies. The conditional-spread rule and migration are internal tooling/maintenance changes.