refactor(core)!: rework agent run lifecycle and status semantics

BREAKING CHANGE: stop() is now async and resolves after the run fully
settles; status decouples from task outcome (new 'stopped' state, LLM
self-reported failure now ends as 'completed'). Lifecycle hooks re-throw
instead of being folded into the result; agent errors go to history.
Adds agent.lastResult.

.vscode/settings.json

@@ -1,5 +1,6 @@
 {
     "cSpell.words": [
+        "agentic",
         "contenteditable",
         "deepseek",
         "historychange",