feat(omo-codex): wire Codex SessionStart telemetry for DAU/WAU/MAU

Adds a new Codex plugin component `telemetry` that emits a single
omo_codex_daily_active event (reason: session_start) from every Codex
SessionStart hook, with the same UTC-day deduplication, hashed
installation identifier, and four-flag opt-out as the install-time
event. Previously omo-codex telemetry only fired on install_completed,
so DAU/WAU/MAU under-reported real Codex usage.

- New plugin component packages/omo-codex/plugin/components/telemetry/
  mirrors the rules/comment-checker/lsp pattern: own src/, tsc build,
  vitest tests, package.json (posthog-node dep), hooks/hooks.json.
- src/codex-hook.ts wraps createPluginPostHog().trackActive(..., "session_start")
  with safeShutdown so Codex session startup never blocks on telemetry.
- Plugin root hooks.json + workspaces register the new component
  alongside rules and ultrawork on SessionStart.
- Aggregate test expectations updated to include the telemetry directory.
- cross-package-equivalence.test.ts pins product-identity constants and
  shouldDisablePostHog behavior to stay byte-equivalent between the CLI
  installer (src/telemetry/) and the plugin runtime
  (plugin/components/telemetry/src/), so the two PostHog sources never
  drift on event name, distinct_id base, dedup file path, or opt-out
  flags.
- PostHogActivityReason union in the CLI-side posthog.ts gains
  "session_start" so future CLI paths can emit the same reason without
  a type break.

packages/omo-codex/plugin/components/telemetry/src/codex-hook.ts

@@ -0,0 +1,49 @@
+import {
+	type PostHogActivityReason,
+	type PostHogClient,
+	createPluginPostHog,
+	getPostHogDistinctId,
+} from "./posthog.js";
+
+export type CodexSessionStartInput = {
+	session_id: string;
+	transcript_path: string | null;
+	cwd: string;
+	hook_event_name: "SessionStart";
+	model: string;
+	permission_mode: string;
+	source: "startup" | "resume" | "clear";
+};
+
+export type CodexTelemetryHookOptions = {
+	createClient?: () => PostHogClient;
+	getDistinctId?: () => string;
+};
+
+const SESSION_START_REASON: PostHogActivityReason = "session_start";
+
+export async function runSessionStartHook(
+	_input: CodexSessionStartInput,
+	options: CodexTelemetryHookOptions = {},
+): Promise<string> {
+	const createClient = options.createClient ?? createPluginPostHog;
+	const getDistinctId = options.getDistinctId ?? getPostHogDistinctId;
+
+	const client = createClient();
+	try {
+		client.trackActive(getDistinctId(), SESSION_START_REASON);
+	} catch {
+		await safeShutdown(client);
+		return "";
+	}
+	await safeShutdown(client);
+	return "";
+}
+
+async function safeShutdown(client: PostHogClient): Promise<void> {
+	try {
+		await client.shutdown();
+	} catch {
+		return;
+	}
+}