test: restore 100% per-file coverage for agent-loop and the acp bridge

agent-loop: behavior tests for retry-while-busy, cancelled recovery
windows, no-facts stream failures, idle-listener preemption, rejected
driver promises under whenIdle, finish-chunk failures after step close,
presentationMeta persistence, pre-aborted and torn-down create/resume
signals, and configured-start failures over existing artifacts or after
teardown. The remaining guards that no public path can reach carry
justified v8 ignore annotations naming the invariant that starves them.

acp bridge: cover the retry-adoption path (a retry turn resolves the
prompt the failed turn deferred), the no-retry quiescence rejection, and
the admission-blocked cancelled settlement; the synchronous send-throw
catch is annotated as a future-proofing guard since the machine's send()
contains listener failures.

packages/acp/acp/src/index.ts

@@ -275,14 +275,16 @@ export function apply(ctx: Context, config: AcpConfig): void {
           record.inflight = inflight
           try {
             record.agent.followup({ content: [{ type: 'text', text }], source: { kind: 'user' } })
+            // The machine's send() contains listener failures and accepts
+            // any typed input; this guards a future synchronous throw so the
+            // slot cannot wedge.
+            /* v8 ignore start -- future-proofing guard, see above */
           } catch (error: unknown) {
             record.inflight = undefined
-            // followup() throws only Errors (invalid input); the String arm
-            // is a defensive fallback for a non-Error throw.
-            /* v8 ignore next */
             const detail = error instanceof Error ? error.message : String(error)
             throw internalError(`prompt was not queued: ${detail}`)
           }
+          /* v8 ignore stop */
           // Admission is pre-turn and retries outlive their failed turn, so a
           // turnless slot settles only at quiescence: a held failure rejects
           // (no retry adopted the prompt); no turn at all means admission