refactor(types): enforce chained-assertion guard in AI and UI (#124361)

refactor(types): enforce chained-assertion guard in AI and UI (#124361)

* refactor(types): drain chained-assertion ledger — ai and ui

* refactor(ui): extract chat state contract

* fix(ai): preserve strict Claude request cleanup

* test(ui): synchronize terminal timeout clock

* fix(ui): validate observer digest before run mutation