feat: peptide-geo-monitor v0.1.0 — AI-visibility dashboard

Add initial plugin implementation per the CTO engineering handoff
(geomonitor/threads/2026-06-ai-visibility-v1/01-cto-eng-handoff.md).

## What
- PGM_Probe_Provider interface + PGM_Probe_Result value object
- PGM_Perplexity_Provider (sonar via OpenRouter/CF AI Gateway)
- PGM_OpenRouter_Provider (GPT-search + Gemini-search, generic parameterised)
- PGM_Gateway_Client: shared HTTP + exponential-backoff retry (mirrors PRAB)
- PGM_Citation_Detector: domain extraction, is_cited(), get_our_position()
- PGM_Cost_Ledger: MTD cost from DB, can_afford() pre-check, hard monthly cap
- PGM_Probe_Runner: peptides×intents×models run, budget abort, in-run cache
- PGM_Table_Manager: pgm_ai_visibility dbDelta, schema-version, drop_table()
- PGM_Config: typed getters, seed_defaults() (12 peptides, 3 intents, 3 models)
- PGM_Cron: weekly pgm_weekly_probe schedule/clear
- PGM_Data_Collector + PGM_Dashboard_Panel interfaces (seam for future panels)
- PGM_Collector_Registry: singleton registry
- PGM_Ai_Visibility_Collector: trendline, standings, MTD cost (score formula)
- PGM_Ai_Visibility_Panel: proxy note, Chart.js trendline, standings table
- PGM_Admin_Page: GEO Monitor menu, Run now POST handler (nonce+capability)
- uninstall.php: DROP TABLE + DELETE pgm_* options + wp_clear_scheduled_hook
- Unit tests (6 files): citation detection, score calc, cost ledger + budget
  abort, provider response parsing (mock HTTP), cron, uninstall purge
- ARCHITECTURE.md, CONVENTIONS.md, CONTEXT.md (score formula), README.md,
  CHANGELOG.md — same-commit per DoD
- CI workflow: PHP lint (8.1/8.2/8.3) + PHPCS + 300-line check

## Security
- Secrets from wp-config constants only; none hardcoded
- manage_options + nonce on every admin action
- Input sanitized; output escaped

Agent-Session: geomonitor v0.1.0

includes/core/class-pgm-plugin.php

@@ -0,0 +1,41 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Main orchestrator for the Peptide GEO Monitor plugin.
+ *
+ * Boots sub-systems in the correct order, hooks into WP, and keeps
+ * the top-level file under the 300-line limit by delegating all logic
+ * to specialist classes.
+ *
+ * Who triggers: plugins_loaded action (peptide-geo-monitor.php).
+ * Dependencies: PGM_Cron, PGM_Admin_Page, PGM_Collector_Registry.
+ *
+ * @see ARCHITECTURE.md — Boot sequence diagram.
+ * @package PeptideGeoMonitor
+ */
+class PGM_Plugin {
+
+	/**
+	 * Wire up all WordPress hooks.
+	 *
+	 * Side effects: Registers admin menus, enqueues assets, and registers the
+	 *               collector/panel registry.
+	 *
+	 * @return void
+	 */
+	public function init(): void {
+		$cron = new PGM_Cron();
+		$cron->register_hooks();
+
+		if ( is_admin() ) {
+			$page = new PGM_Admin_Page();
+			$page->register_hooks();
+		}
+
+		// Register the v1 AI-visibility collector + panel.
+		$registry = PGM_Collector_Registry::instance();
+		$registry->register_collector( new PGM_Ai_Visibility_Collector() );
+		$registry->register_panel( 'ai_visibility', new PGM_Ai_Visibility_Panel() );
+	}
+}