fix(runtime): keep PTY tool results JSON-safe

[Astro-Han]

Summary

• Omit lastAlternateScreen from projected PTY output when it has no model-visible text instead of materializing the optional field as undefined.
• Cover a stopped PTY background task result at the durable RuntimeEvent serialization boundary.

Verification

• node --test packages/runtime/dist/__tests__/shell-run-tool-result.test.js (9 passed)
• npm run format:check
• npm run lint
• npm test -w @maka/runtime (3,156 passed, 9 skipped; 2 unrelated deep-research tests could not open a stale local storage build reporting schema 10 as newer than supported schema 9)

Root cause

projectPtyOutputForModel produced an own lastAlternateScreen: undefined property when a PTY had no alternate-screen text. The durable RuntimeEvent writer correctly rejects nested undefined because it cannot be represented losslessly in JSON, so a successful StopBackgroundTask side effect failed at the T2 outcome commit and left the turn failed.