refactor: notify/footer 持久配置收编到 config/，chrome bridge 地址固定

仓库规则要求 config/ 为唯一配置来源、不以环境变量作持久默认值：
- pi-notify 改读 config/pi-notify.json（逐字段校验回退，复用原 parsers 钳制）
- pi-minimal-footer 改读 config/pi-minimal-footer.json
- pi-chrome 移除 PI_CHROME_BRIDGE_HOST/PORT，固定 127.0.0.1:17318
  （浏览器端 service_worker.js 硬编码同一地址，单侧可配只会断连）

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config/pi-notify.json

@@ -0,0 +1,11 @@
+{
+  "enabled": true,
+  "title": "π - {project}",
+  "body": "{message}",
+  "messageSource": "assistant",
+  "messageMax": 80,
+  "minSeconds": 3,
+  "visibility": "always",
+  "action": "focus",
+  "soundCommand": ""
+}