feat(images): expose rewriter system prompt as admin setting

CEO feedback: every generated image is anchoring on the cartoon
peptide-with-arms-and-legs character because the hardcoded rewriter
system prompt both lists "anthropomorphized molecules" first in its
primer and ships a few-shot example that literally shows one. The ask
is to give CEO the lever without a code change.

Adds `prautoblogger_image_prompt_instructions` (textarea) in the
Images section. Default value is the existing REWRITER_SYSTEM_PROMPT
constant verbatim, so behaviour is unchanged on upgrade until someone
edits it. `Image_Prompt_Builder::rewrite_via_llm()` reads the option
at call time via a new `resolve_system_prompt()` helper and falls back
to the constant if the option is blank or whitespace-only, so a saved
empty value cannot brick image generation.

Const visibility flipped from `private` to `public` so the settings
field can reference it directly as the field default.

Tests:

- `test_rewrite_uses_setting_when_present` — option set, assert the
  LLM's system message equals the option content.
- `test_rewrite_falls_back_to_default_when_setting_empty` — option
  whitespace, assert the LLM's system message equals the constant.

Uninstall: new option covered by the existing prautoblogger_% LIKE
sweep in uninstall.php.

Cost impact: none by default. Future CEO edits will change prompt
length; current default is ~180 tokens, budget impact linear from
there.

Part of the v0.8.0 image bundle (thread image-mime-bug, message 03).

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

includes/admin/class-settings-fields-extended.php

@@ -256,6 +256,14 @@ class PRAutoBlogger_Settings_Fields_Extended {
 				'default'     => PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX,
 				'description' => __( 'Appended to every image prompt. Controls visual look. Changing mid-run causes visible style drift.', 'prautoblogger' ),
 			],
+			[
+				'id'          => 'prautoblogger_image_prompt_instructions',
+				'label'       => __( 'Image Prompt Instructions', 'prautoblogger' ),
+				'type'        => 'textarea',
+				'section'     => 'prautoblogger_images',
+				'default'     => PRAutoBlogger_Image_Prompt_Builder::REWRITER_SYSTEM_PROMPT,
+				'description' => __( 'System prompt given to the rewriter LLM that turns each article into a SCENE + CAPTION for the image generator. Changing this reshapes the look of all future images. Leave blank to use the default.', 'prautoblogger' ),
+			],
 			[
 				'id'          => 'prautoblogger_image_nsfw_retry',
 				'label'       => __( 'Retry NSFW-Blocked Images', 'prautoblogger' ),