feat(B1): persist request_json on every chat LLM call via process-scoped recorder

The OpenRouter provider stashes the outgoing request BODY (never headers --
Authorization is built separately and build_body() copies only whitelisted
option keys) in the new PRAutoBlogger_Request_Recorder before dispatch;
Cost_Tracker::log_api_call() consumes it (consume-once) into
generation_log.request_json. All seven chat call sites covered with zero
call-site changes; error/timeout rows carry the request too. Retention
rides the existing prautoblogger_request_json_retention_days prune.

Unblocks M3 edit+rerun (stage inputs now exist) and makes the M2 dossier
raw-input trace real (QA seq 20 F1).

Agent-Session: engineer-prab-m3-20260612

includes/core/class-cost-tracker.php

@@ -69,7 +69,9 @@ class PRAutoBlogger_Cost_Tracker {
 	/**
 	 * Log an API call with its token usage and estimated cost.
 	 *
-	 * Side effects: database insert into prab_generation_log.
+	 * Side effects: database insert into prab_generation_log. Consumes the
+	 * Request_Recorder stash (v0.20.0 / B1) into the row's request_json so
+	 * every chat call's input is persisted (retention-pruned after R days).
 	 *
 	 * @param int|null    $post_id           WordPress post ID (null during generation, set later).
 	 * @param string      $stage             Pipeline stage (see PRAutoBlogger_Stage_Display_Map).
@@ -117,6 +119,7 @@ class PRAutoBlogger_Cost_Tracker {
 				'estimated_cost'    => $cost,
 				'response_status'   => $response_status,
 				'error_message'     => $error_message,
+				'request_json'      => PRAutoBlogger_Request_Recorder::consume(),
 				'agent_role'        => $agent_role ?? PRAutoBlogger_Stage_Display_Map::default_agent_role( $stage ),
 				'prompt_version'    => $this->resolve_prompt_version( $stage, $prompt_key ),
 				'created_at'        => current_time( 'mysql' ),

includes/core/class-request-recorder.php

@@ -0,0 +1,73 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * phpcs:ignore WordPress.Files.FileName.InvalidClassFileName -- class naming convention differs from WordPress standard
+ *
+ * Per-PHP-process stash for the most recent outgoing LLM request body.
+ *
+ * What: A process-scoped static slot recording the JSON request body the
+ *       OpenRouter provider is about to dispatch, so Cost_Tracker::
+ *       log_api_call() can persist it to `generation_log.request_json`
+ *       (v0.20.0 / B1) without threading the body through every call
+ *       site. Mirrors the Run_Context precedent (v0.18.0).
+ *
+ *       SECURITY GUARANTEE: only the request BODY is ever recorded.
+ *       Authorization headers are built separately
+ *       (Request_Builder::build_headers()) and can never reach this
+ *       class; build_body() copies only whitelisted option keys.
+ *
+ *       Consume-once semantics: consume() clears the slot, and record()
+ *       overwrites it at every dispatch — so a stale body from an
+ *       unlogged failed call can never attach to a later, unrelated
+ *       log row.
+ * Who triggers it: OpenRouter_Provider::send_chat_completion() (record,
+ *       pre-dispatch so error rows carry the request too);
+ *       Cost_Tracker::log_api_call() (consume).
+ * Dependencies: none.
+ *
+ * @see providers/class-open-router-provider.php — Records before dispatch.
+ * @see core/class-cost-tracker.php              — Consumes into the log row.
+ * @see core/class-run-context.php               — The pattern this mirrors.
+ * @see ARCHITECTURE.md #24                      — Edit + re-run substrate (B1).
+ */
+class PRAutoBlogger_Request_Recorder {
+
+	/** @var string|null JSON-encoded request body of the in-flight LLM call. */
+	private static ?string $request_json = null;
+
+	/**
+	 * Record the outgoing request body for the next log row.
+	 *
+	 * Overwrites any previous stash (one in-flight chat call per PHP
+	 * process). Bodies that fail JSON encoding are skipped silently —
+	 * the log row's request_json stays NULL, never blocking the call.
+	 *
+	 * @param array<string, mixed> $body Request body from Request_Builder::build_body().
+	 * @return void
+	 */
+	public static function record( array $body ): void {
+		$encoded            = wp_json_encode( $body );
+		self::$request_json = is_string( $encoded ) ? $encoded : null;
+	}
+
+	/**
+	 * Return the stashed request body and clear the slot (consume-once).
+	 *
+	 * @return string|null JSON request body, or null when nothing is stashed.
+	 */
+	public static function consume(): ?string {
+		$json               = self::$request_json;
+		self::$request_json = null;
+		return $json;
+	}
+
+	/**
+	 * Clear the stash (tests / defensive teardown).
+	 *
+	 * @return void
+	 */
+	public static function clear(): void {
+		self::$request_json = null;
+	}
+}