feat: OPENCODE_TOOL_CHOICE=required for agent tool binding

[TimothyVang]

Summary

• OPENCODE_TOOL_CHOICE=required / VERDICT_FORCE_TOOL_CHOICE=1 forces OpenAI-compatible tool_choice: required on non-final agent steps.
• Enables agent-path seal with tool-capable local models (verified: gpt-oss:20b → used_fallback=0).

Evidence (operator host)

PASS: sealed with ed25519 (agent path; used_fallback=0)
signature_kind=ed25519 overall=true used_fallback=0
model=gpt-oss:20b OPENCODE_TOOL_CHOICE=required

Test plan

• bun test packages/core/test/tool-choice-flag.test.ts
• rebuild binary + live spark seal smoke

[github-actions]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve a final turn when forcing tool choice

When OPENCODE_TOOL_CHOICE=required or VERDICT_FORCE_TOOL_CHOICE=1 is set for an agent without a configured steps limit (the built-in build agent leaves steps unset), isLastStep never becomes true, so every provider turn is sent with toolChoice: "required". Any local tool call then sets needsContinuation and the runner starts another turn that again cannot produce a final text answer, causing default sessions to keep issuing tools until an error/context limit instead of settling. Only force required when there is a finite final step or otherwise disable it after tool settlement.

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