feat(agent): capability registry, Pi AgentRuntimePort, fake model provider and redaction guard (W1-5)

- CapabilityRegistry: descriptor(id/version/schema/scope/risk/idempotent/preview/replay),
  invoke = scope limit -> input schema -> server-side authorize -> idempotency key -> handler
  -> output schema -> audit; capability id <-> tool name mapping (download.submit <-> download__submit)
- PiAgentRuntime (pi-agent-core AgentHarness 0.87.1): whitelist tools + before_tool re-check,
  redacted event stream, JSONL session at sessionDir/<runId>.jsonl, resume/abort/compact,
  tool-call/duration/token/cost budgets -> abort + run_failed(budget)
- AgentSettings -> pi-ai createProvider (anthropic / openai-compatible), compaction mapping
- FakeModelProvider + createFakeRuntime (scripted, transcript-derived, survives crash/resume)
- buildSystemPrompt, redactForModel; 27 tests incl. SIGKILL + resume

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/agent/src/runtime/event-channel.ts

@@ -0,0 +1,40 @@
+/** 追加式事件通道:全部事件保存在内存,支持多个消费者按 seq 回放 + 实时跟随 */
+export class EventChannel<T> {
+  private readonly items: T[] = [];
+  private waiters: Array<() => void> = [];
+  private closed = false;
+
+  push(item: T): void {
+    if (this.closed) return;
+    this.items.push(item);
+    this.wake();
+  }
+
+  close(): void {
+    this.closed = true;
+    this.wake();
+  }
+
+  get size(): number {
+    return this.items.length;
+  }
+
+  snapshot(): T[] {
+    return [...this.items];
+  }
+
+  async *iterate(fromIndex = 0): AsyncGenerator<T> {
+    let i = Math.max(0, fromIndex);
+    while (true) {
+      while (i < this.items.length) yield this.items[i++] as T;
+      if (this.closed) return;
+      await new Promise<void>((resolve) => this.waiters.push(resolve));
+    }
+  }
+
+  private wake(): void {
+    const waiters = this.waiters;
+    this.waiters = [];
+    for (const w of waiters) w();
+  }
+}