test: close per-file coverage gaps opened by the message-machine refactor

Downstream packages lost the tests that exercised their agent-loop-facing
edges when the loop was rewritten. Restore 100% per-file coverage with
behavior tests through public seams: llm-retry config validation and
cancellation races, goal replay drift/staleness/teardown edges, plan-mode
disposed-flush, workspace-context empty-change commits, api-proxy
synchronous send failures, acp-snapshot spill-path extraction and refresh
write-back, ACP injection-triggered turns, cli-demo and tui inbox
lifecycle edges, and agent-loop retry/settlement/lifecycle branches.
The only source changes are narrowly-justified v8 ignore annotations on
invariant guards and one redundant-guard removal (workspace-context).

packages/examples/cli-demo/src/cli.ts

@@ -241,6 +241,10 @@ export async function runOneShot(ctx: Context, options: OneShotOptions): Promise
     resolveTurn()
   }
   const settleRejected = (error: Error): void => {
+    // The once-registered abort listener is the only rejecter, and a settled
+    // prompt makes targetTurn defined so onAbort skips rejection entirely;
+    // kept for symmetry with settleResolved.
+    /* v8 ignore next -- unreachable second settlement, see above */
     if (firstTurnEnded) return
     firstTurnEnded = true
     rejectTurn(error)