refactor(runtime): remove the unused AgentRun.execute execution path #3034

Problem

AgentRun currently exposes two ways to execute a run:

1. AgentRun.execute() constructs and drives AiSdkFlow itself.
2. The production path uses RuntimeKernel, which calls AgentRun.begin() and owns flow execution, lifecycle, cancellation, steering, and finalization.

The first path has no production caller and is not exported through the Runtime package entry point. Only four direct calls in session-manager-terminal-ledger.test.ts exercise it.

Keeping both paths creates two execution authorities inside Runtime. AgentRun.execute() also reconstructs invocation state and hard-codes source: 'desktop', so future lifecycle changes can drift from the Runtime Kernel path without affecting production tests.

Proposed change

• Remove AgentRun.execute().
• Keep AgentRun.begin(), event acceptance, persistence, failure recording, and finalization as the lifecycle seam consumed by RuntimeKernel.
• Rewrite the affected tests against the production execution path or narrower AgentRun lifecycle methods.
• Remove imports and helpers used only by execute().

Acceptance criteria

• AgentRun.execute() no longer exists.
• Runtime has one production execution path through RuntimeKernel.
• The affected terminal-ledger behavior remains covered without a test-only execution authority.
• No externally observable Runtime behavior changes.
• Focused Runtime tests, type checking, and formatting pass.

Non-goals

• Changing AiSdkFlow behavior.
• Changing RuntimeEvent or SessionEvent semantics.
• Moving persistence or terminal-state ownership out of AgentRun.
• Exporting AgentRun as a public API.

Context

Verified on main at 1a71db5c0fa85771330354360388ecf47ba95c2e.

Relevant code:

• packages/runtime/src/agent-run.ts
• packages/runtime/src/runtime-kernel.ts
• packages/runtime/src/__tests__/session-manager-terminal-ledger.test.ts

This issue was identified and prepared with AI assistance (Codex). A human contributor reviewed the analysis and owns the submission.