feat(diagnostics): funnel events throughout setup with persisted install-id

Shared bash + node emitter in setup/lib/diagnostics.{sh,ts} reads/writes data/install-id so every event from a single install shares one distinct_id — bash-side setup_launched/setup_start, node-side auto_started, per-step started/completed, auth_method_chosen, channel_chosen, first_chat_ready/failed, setup_incomplete, setup_aborted, setup_completed. Opt-out via NANOCLAW_NO_DIAGNOSTICS=1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/lib/diagnostics.ts

@@ -0,0 +1,70 @@
+/**
+ * Thin PostHog emitter shared across setup:auto code. Fire-and-forget —
+ * never throws, never blocks. Reuses data/install-id (same file bash
+ * uses in setup/lib/diagnostics.sh) so events from the bash and node
+ * halves of a single install join into one funnel.
+ *
+ * Honors NANOCLAW_NO_DIAGNOSTICS=1.
+ */
+import { randomUUID } from 'crypto';
+import fs from 'fs';
+import path from 'path';
+
+const POSTHOG_KEY = 'phc_fx1Hhx9ucz8GuaJC8LVZWO8u03yXZZJJ6ObS4yplnaP';
+const POSTHOG_URL = 'https://us.i.posthog.com/capture/';
+const INSTALL_ID_PATH = path.join('data', 'install-id');
+
+let cached: string | null = null;
+
+export function installId(): string {
+  if (cached) return cached;
+  try {
+    const existing = fs.readFileSync(INSTALL_ID_PATH, 'utf-8').trim();
+    if (existing) {
+      cached = existing;
+      return existing;
+    }
+  } catch {
+    // fall through to create
+  }
+  const id = randomUUID().toLowerCase();
+  try {
+    fs.mkdirSync(path.dirname(INSTALL_ID_PATH), { recursive: true });
+    fs.writeFileSync(INSTALL_ID_PATH, id);
+  } catch {
+    // best-effort; still return the id so the event fires
+  }
+  cached = id;
+  return id;
+}
+
+export function emit(
+  event: string,
+  props: Record<string, string | number | boolean | undefined> = {},
+): void {
+  if (process.env.NANOCLAW_NO_DIAGNOSTICS === '1') return;
+
+  const cleaned: Record<string, unknown> = { platform: process.platform };
+  for (const [k, v] of Object.entries(props)) {
+    if (v === undefined) continue;
+    cleaned[k] = v;
+  }
+
+  const body = JSON.stringify({
+    api_key: POSTHOG_KEY,
+    event,
+    distinct_id: installId(),
+    properties: cleaned,
+  });
+
+  const ctrl = new AbortController();
+  const timer = setTimeout(() => ctrl.abort(), 3000);
+  void fetch(POSTHOG_URL, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body,
+    signal: ctrl.signal,
+  })
+    .catch(() => {})
+    .finally(() => clearTimeout(timer));
+}