fix(tui): prevent prompt submit storms

[YOMXXX]

Summary

• add a single-flight guard around normal prompt submissions
• block duplicate prompt submits until the session leaves busy state and returns idle
• release the guard on dispatch failure or if no run status arrives after dispatch

This mitigates terminal paste fallbacks that send multiline clipboard content as a stream of Return key events, which could otherwise submit each pasted line as a separate prompt.

Fixes #939

Tests

• bun test test/cli/cmd/tui --timeout 30000
• bun typecheck
• git diff --check