feat(kap-server): add steer flag to submit a prompt directly into the running turn

[sailist]

Related Issue

Internal feature request (no linked issue).

Problem

API clients that want to steer a fresh message into the active turn must use a two-step flow: submit a prompt (which lands in the queue on a busy session), then call POST /api/v1/sessions/{session_id}/prompts:steer with the returned prompt id. The engine (IAgentPromptService.submitSteer) and the internal node-sdk RPC lane (session.steer) already support one-shot steering, but the public REST surface does not.

What changed

• POST /api/v1/sessions/{session_id}/prompts accepts a new optional steer boolean. With steer: true, a prompt submitted while the session is busy is steered directly into the running turn — the one-call form of submitting and then steering; on an idle session the prompt starts a new turn as usual. The submit response keeps its shape and reports the prompt's real state (running / queued).
• A race where the active turn ends between submission and steering is tolerated the same way the engine's submitSteer handles it: the prompt simply stays queued instead of failing the request.
• steer combined with skills is rejected with 40001, matching the existing prompt_id + skills rule.
• The parameter is documented in docs/en/reference/server-api.md and docs/zh/reference/server-api.md.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

⚠️ No Changeset found

Latest commit: e71090d

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commit: e71090d

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add a changeset for the new REST capability

This introduces and documents a user-visible REST API capability, but the commit contains no .changeset/ entry. Without one, the release changelog will omit the new steer option; add a user-facing changeset for the shipped CLI package as required by the repository workflow.

AGENTS.md reference: AGENTS.md:L86-L88

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[sailist]

No changeset for this one: the steer flag is an additive, opt-in parameter on the server-side protocol surface — it changes no existing behavior and is not perceivable by CLI end users, consistent with previous server-protocol additions in this repo.