feat(core): publish interrupted session steps

[kitlangton]

Summary

• publish durable session.next.step.interrupted outcomes separately from failures
• project interrupted assistant steps with a typed interrupted finish
• enforce exactly one terminal settlement when interruption races provider completion
• expose the event through generated clients

Verification

• bun typecheck
• bun test test/session-runner.test.ts test/session-runner-message.test.ts in packages/core
• bun test test/event-manifest.test.ts in packages/schema
• client and SDK generation
• simplify review: reuse, quality, and efficiency passes