fix: use completed history for forked subagents

[K-Mistele]

Summary

• make public fork: true subagents persist the completed-history selector
• apply the same safe default to legacy fork records when they are projected or resumed
• cover both paths so child requests omit the unresolved parent tool call that launched them

Why

A fork starts before its parent subagent tool call can have a result. Including that parent call in the child request sends OpenAI a function call with no output, which it rejects.

Validation

• bun --bun x vitest run packages/fold-core/test/Subagents/SubagentFork.vi.test.ts packages/fold-core/test/Subagents/SubagentToolWire.vi.test.ts packages/fold-core/test/Projection/Projection.vi.test.ts
• bun --bun --cwd packages/fold-core test
• bun --bun run typecheck --filter @humanlayer/fold-core
• bun --bun run format:check
• bun --bun run lint (0 errors; existing warnings only)

No changeset: this repository has no .changeset configuration and the directly modified @humanlayer/fold-core package is private.