Add workflow idempotency keys so active runs deduplicate

[ericclemmons]

tl;dr – Add active-lifetime idempotencyKey support to runWorkflow() so concurrent callers reuse one Workflow instance.

Agents currently persist Workflow tracking only after workflow.create() returns. Concurrent callers can therefore both create instances before either tracking row exists, and callers that compensate with blockConcurrencyWhile() hold a Durable Object lock across external I/O.

This change atomically claims (workflow_name, idempotency_key) in Agent SQLite before the Workflow RPC. Calls with an occupied key return the active Workflow ID. Terminal tracking rows remain retained, while their key becomes available for a new Workflow instance.

The proposed RFC keeps queue, cancel, and replace concurrency policies out of this initial primitive because they require durable payload storage and lifecycle orchestration.

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References: Durable Objects concurrency guidance

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🦋 Changeset detected

Latest commit: 28c4354

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
agents	Minor
@cloudflare/agent-think	Patch

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