refactor(core): simplify session input promotion

[kitlangton]

Summary

• keep session.next.prompt.admitted as the durable, client-visible record of pending Session input
• replace the redundant session.next.prompt.promoted event with the existing session.next.prompted event when input becomes model-visible
• atomically consume the inbox row and project the user message from Prompted, regenerate SDK/OpenAPI output, and reset disposable V2 state while preserving canonical V1 conversations

Design

The event log retains both facts needed by servers and clients:

• PromptAdmitted: the prompt was accepted and is pending; replay can rebuild the inbox and clients can display queued input
• Prompted: the runner made that input model-visible; the projector marks the inbox row consumed and appends the user message in the same EventV2 transaction

Execution ownership remains process-local. Future multi-server execution still requires an explicit lease, but accepted pending work remains replayable and projectable onto another database.

Verification

• bun run test test/session-prompt.test.ts test/session-projector.test.ts test/session-create.test.ts test/session-runner.test.ts test/session-runner-recorded.test.ts test/database-migration.test.ts (145 passed)
• bun test test/cli/tui/data.test.tsx (7 passed)
• bun run typecheck (23/23 workspace tasks)
• bun run script/migration.ts --check
• revised simplify reuse, quality, and efficiency reviews completed

The earlier full Core suite passed 1005/1006 tests. The unrelated macOS watcher test consistently received an add event for .git/HEAD where it expects change; the focused watcher rerun reproduced the same failure.