chore(prompts): drop unused target_platform supplement

The target-platform supplement only fed legacy prompt builders that read
the now-removed wizard_state platforms field. Drop the YAML entry,
loader struct field, and runtime helper.

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

deploy/k3s/config/prompts.yml

@@ -210,7 +210,6 @@ runtime:
   prompt_rule_scene_supplement_format: "适用场景：%s"
   prompt_rule_tone_supplement_format: "默认语气：%s"
   prompt_rule_word_count_supplement_format: "建议字数：%d 字左右"
-  prompt_rule_target_platform_supplement_format: "目标发布平台：%s"
   prompt_rule_supplement_heading: "补充要求："
   prompt_rule_output_requirements_section: |
     输出要求：

deploy/prompts.yml

@@ -210,7 +210,6 @@ runtime:
   prompt_rule_scene_supplement_format: "适用场景：%s"
   prompt_rule_tone_supplement_format: "默认语气：%s"
   prompt_rule_word_count_supplement_format: "建议字数：%d 字左右"
-  prompt_rule_target_platform_supplement_format: "目标发布平台：%s"
   prompt_rule_supplement_heading: "补充要求："
   prompt_rule_output_requirements_section: |
     输出要求：