refactor: harden async runtime and UI boundaries

[sudo-tee]

Summary

• Harden server startup, API calls, streaming subscriptions, promises, timers, and event batching.
• Prevent stale callbacks and events from crossing server, session, or workspace boundaries.
• Make snapshot and Git review operations asynchronous, context-aware, and serialized.
• Preserve multiline prompts through JSONL history persistence with atomic rewrites.
• Decouple prompt controllers and session selection from UI orchestration.
• Improve output folding, scrolling, completion fallback, and curl response parsing.
• Add regression coverage and document performance findings.

Performance

Event collapsing avoids repeated scans of intervening events:

• 1,000 updates: 1.49 ms -> 0.67 ms
• 4,000 updates: 18.05 ms -> 2.33 ms
• 8,000 updates: 78.79 ms -> 4.80 ms