docs: introduce Beta/Stable two-channel branching workflow (#327)

把分支模型从「dev/main」改造为「Beta/正式版」双渠道，渠道语义直接由分支名承载：

- `beta` — Beta 渠道（开发版），默认分支、集成缓冲区，所有 PR 一律打到这里。
  Beta 包不推送给普通用户，只面向主动加入 Beta 渠道的用户。
- `main` — 正式版渠道（Stable），始终可发布。仅由维护者在双端冒烟测试通过后从
  beta 合入；普通用户默认拿到的就是这条线。

文档与 CI 改动：
- README.md / README.zh.md：把「Contributing workflow / 贡献流程」章节
  改写为 Beta/正式版双渠道，含分支流向 ASCII 图、6 条核心规则，
  以及「Beta 不溢出正式版」的隔离要求。
- CLAUDE.md：把 `### Branch & contribution workflow` 改名为
  `### Branch & release-channel workflow`，明确「分支名=渠道名」。
- ci.yml：触发分支从 [main, dev] 改为 [main, beta]。

仓库侧已同步：
- 远端 dev 分支已重命名为 beta（git push origin dev:beta + 删除旧 dev）
- GitHub 仓库默认分支已切到 beta
- release-tauri.yml 用 v*-tauri tag 触发，跟分支无关，发版机制不受影响

Beta 包的 opt-in 分发机制（per-channel updater endpoint + 设置页开关）
将在独立 PR 中接入。在它落地之前，所有 v*-tauri tag 都按正式版对待，
不要从 beta 分支直接打 tag。

Co-authored-by: baiqing <lbx12309@icloud.com>

README.md

@@ -208,6 +208,37 @@ Logs: `~/Library/Logs/OpenLess/openless.log` (macOS) / `%LOCALAPPDATA%\OpenLess\
 
 **Windows build** — see [`openless-all/README.md`](openless-all/README.md) for MSVC vs GNU/MinGW routes.
 
+## Contributing workflow
+
+OpenLess uses a two-channel branching model.
+
+- **`beta`** — the **Beta channel**. Default branch and integration buffer; all in-progress development lands here. Beta builds may exist but are **not pushed to regular users** — they only reach people who explicitly opt into the Beta channel.
+- **`main`** — the **Stable channel (正式版)**. Always-releasable. The build everyone gets by default.
+
+```text
+your fork / topic branch
+        │  (test locally on your target platform first)
+        ▼
+   PR → beta  ← AI review (one pass, advisory only)
+        │     ← maintainer lightweight glance (scope, cross-module impact)
+        ▼
+       merged into beta
+        │  (periodically, after a two-platform smoke build)
+        ▼
+       merged into main  →  tag `v<version>-tauri`  →  release CI → Stable users
+```
+
+Rules of thumb:
+
+- **Open PRs against `beta`, never against `main`.** GitHub already defaults the base branch to `beta` for new PRs.
+- **Verify the change on your target platform before opening the PR** — build green is necessary, manual verification is required.
+- **AI review runs once per PR and is advisory.** Don't loop on it. Apply your judgment.
+- **Keep AI rework rounds tight (1–2).** If a fix resists, ask a human or restart with fresh context — multi-round AI back-and-forth tends to do more harm than good here.
+- **Beta work must not leak to Stable.** `main` only receives merges from `beta`, performed by maintainers after a successful two-platform smoke build. No direct pushes to `main`.
+- **Stable releases are cut from `main`** by pushing a `v<version>-tauri` tag — see the maintainer release checklist below.
+
+Beta release distribution (opt-in, not yet wired): Beta builds are intended for users who consciously join the Beta channel; the in-app updater currently treats every release as Stable, and a follow-up change will introduce per-channel updater endpoints + a Settings toggle.
+
 ## Credentials
 
 Credentials live in the OS credential vault (service = `com.openless.app`): macOS Keychain, Windows Credential Manager, or Linux keyring. A legacy plaintext JSON file is read only as a migration source and removed after a successful vault write: