Add atomic agent turn start with continuation resolution

[ajac-zero]

Problem

An external agent runtime currently needs separate continuation lookup, turn creation, user-item append, and replay calls. Those calls leave races around ownership, predecessor validity, concurrent continuations, and conversation edits.

Proposed change

Add an agent-facing atomic start operation, for example POST /v1/agent-turns, that in one transaction:

• authenticates the tenant, owner, and agent deployment
• resolves an optional previous_response_id
• returns the same not-found result for absent and wrong-owner predecessors
• verifies the predecessor is terminal and continuable
• enforces a linear/head continuation policy for the initial version
• creates a conversation when no predecessor is supplied
• creates a pending turn
• appends the ordered user input items
• reserves the runtime-supplied response_id
• records and validates an idempotency request digest
• returns the conversation ID, turn ID, predecessor sequence boundary, and current input boundary

An exact retry must return the original result. Reusing the key with different input must return a typed conflict. An unknown HTTP outcome must be reconcilable by the same idempotency key or response ID.

Acceptance criteria

• Start is one database transaction.
• Concurrent starts from the same linear head produce one winner and one typed conflict.
• No model execution is required before the operation commits.
• Existing-conversation and new-conversation paths are covered.
• Exact retry, altered retry, wrong owner, wrong agent, missing predecessor, non-terminal predecessor, and concurrent continuation are tested.
• The wire contract and failure codes are documented.

Integration context

This is the durable admission boundary needed by Bonsai before it exposes a new Open Responses resp_* ID.

[dispatch-developer]

Triage rationale: this spans a transactional API boundary, persistence/idempotency design, concurrency control, wire documentation, and a broad test matrix; it is substantial but bounded.

[dispatch-developer]

Medium: the transactional admission path and its concurrency/idempotency guarantees span API, persistence, and targeted integration tests, but the scope is well-defined.