Opik Wave 1 Commit 3: Version bump, docs, and hook registration

Version & Changelog:
- prautoblogger.php: 0.11.0 → 0.12.0
- Add Opik PHP constants (PRAUTOBLOGGER_OPIK_API_KEY, WORKSPACE, URL_OVERRIDE)
  with fallback empty defaults so wp-config.php can override
- CHANGELOG.md: v0.12.0 section describing Wave 1 scope, feature flag,
  observability benefits, cost model, and Wave 2 preview

Documentation:
- ARCHITECTURE.md: New 'Opik LLM Observability (Optional)' section with
  feature-flag design, credential isolation, span lifecycle, call sites,
  cost/performance implications, and code structure overview
- CONVENTIONS.md: Step-by-step guide 'Instrumenting a New LLM Call Site'
  with code examples, key rules (no full prompts, async dispatch, separate
  cost tracking), and minimal instrumentation pattern

Hook Registration:
- class-prautoblogger.php: Register 'prautoblogger_opik_dispatch' cron action
  * Instantiates Opik_Client, Opik_Span_Queue, Opik_Dispatcher
  * Checks credentials and calls dispatcher->dispatch()
  * Updates 'prautoblogger_opik_last_dispatch' option for admin status display
- Instantiate Opik_Settings in admin_init for admin UI registration

All changes preserve ≤ 300-line files.

Part of Wave 1 (observability skeleton).
Target: v0.12.0

includes/class-prautoblogger.php

@@ -116,6 +116,30 @@ class PRAutoBlogger {
 		$registry = $this->executor->get_model_registry();
 		add_action( 'prautoblogger_refresh_model_registry', array( $registry, 'refresh' ) );
 
+		// Opik observability: async trace/span dispatch.
+		add_action(
+			'prautoblogger_opik_dispatch',
+			static function (): void {
+				if ( ! defined( 'PRAUTOBLOGGER_OPIK_API_KEY' ) || empty( PRAUTOBLOGGER_OPIK_API_KEY ) ) {
+					return;
+				}
+				$client = new PRAutoBlogger_Opik_Client(
+					PRAUTOBLOGGER_OPIK_API_KEY,
+					PRAUTOBLOGGER_OPIK_WORKSPACE,
+					PRAUTOBLOGGER_OPIK_URL_OVERRIDE
+				);
+				$queue = new PRAutoBlogger_Opik_Span_Queue();
+				$dispatcher = new PRAutoBlogger_Opik_Dispatcher( $client, $queue );
+				$dispatcher->dispatch();
+				update_option( 'prautoblogger_opik_last_dispatch', time() );
+			}
+		);
+
+		// Opik settings registration.
+		if ( is_admin() ) {
+			new PRAutoBlogger_Opik_Settings();
+		}
+
 		// Single-idea generation from the Ideas browser page.
 		add_action( 'prautoblogger_generate_from_idea', array( 'PRAutoBlogger_Ideas_Browser', 'on_cron_generate_from_idea' ) );