Add codex2api as a protocol-first OAuth source

This keeps the existing CPA, SUB2API, and contribution flows intact
while introducing codex2api as a separate source that plugs into
Step 7 and Step 10 through backend APIs instead of page DOM.

The sidepanel now exposes codex2api settings, preserves the built-in
local default through placeholder-only UI, and accepts user-provided
public admin URLs. Step 7 now treats missing or invalid management
secrets as terminal config errors so the flow stops instead of retrying
needlessly.

Constraint: New source must be additive and must not break existing CPA/SUB2API/contribution paths
Rejected: Drive codex2api through admin page button clicks | too brittle against frontend DOM changes
Rejected: Reuse contribution mode as a source surface | violates existing panelMode/runtime-mode boundary
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep codex2api on the protocol path unless its API becomes insufficient; do not add a panel content script without proving the protocol path cannot work
Tested: npm test; targeted codex2api/auth retry loops repeated 40 times; full suite repeated 5 times
Not-tested: Real browser run against a live codex2api instance with a real Admin Secret and OpenAI authorization

项目开发规范（AI协作）.md

@@ -82,6 +82,14 @@
 5. 是否需要成功收尾逻辑
 6. 是否需要 README 与完整链路文档更新
 
+补充约定：
+
+- 如果新增来源本身已经提供稳定的后台协议接口，可以直接走协议分支接入：
+  - 步骤 7 通过 `background/panel-bridge.js` 生成 `auth_url`
+  - 步骤 10 通过 `background/steps/platform-verify.js` 直接提交 localhost callback
+- 这类来源优先复用现有 OpenAI 授权页与 localhost callback 主链，不要为了“看起来统一”再额外新增一套页面 DOM 自动点击内容脚本。
+- 只有当目标来源没有可用协议接口、必须依赖后台页面按钮时，才新增对应的 panel content script。
+
 ### 3.2.1 共享别名邮箱逻辑补充
 
 当 Gmail / 2925 这类“既影响注册邮箱生成，又影响 sidepanel 表单行为”的 provider 发生变化时，必须优先检查是否应落入共享层，而不是继续把规则分散写在：
@@ -119,7 +127,7 @@
 当前约定示例：
 
 - `contributionMode` 是 sidepanel 的运行态 UI 模式，不是新的 `panelMode`
-- `panelMode` 仍然只允许 `cpa | sub2api`
+- `panelMode` 当前允许 `cpa | sub2api | codex2api`
 - 运行态模式不能混进 `PERSISTED_SETTING_DEFAULTS`
 - 运行态模式不能混进配置导入/导出
 - 如果运行态模式会临时覆盖某些持久配置的显示值，必须同时处理好“退出模式后恢复”和“自动保存不能误覆盖原配置”这两个问题