feat: add Contributor License Agreement (CLA) and signing workflow

Introduce an individual CLA (license-grant style, based on Apache ICLA
v2.2) with English as the authoritative text and a Chinese reference
translation. Contributors sign by replying to a bot comment on their
first PR; signatures are recorded in the langbot-app/cla repository
and cover all repositories in the organization.

- CLA.md: agreement text (grantee: Beijing Langbo Intelligent
  Technology Co., Ltd.)
- .github/workflows/cla.yml: contributor-assistant action pinned to
  v2.6.1, signatures stored remotely in langbot-app/cla
- CONTRIBUTING.md / PR template: bilingual CLA notice

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/pull_request_template.md

@@ -21,6 +21,7 @@
 *请在方括号间写`x`以打勾 / Please tick the box with `x`*
 
 - [ ] 阅读仓库[贡献指引](https://github.com/langbot-app/LangBot/blob/master/CONTRIBUTING.md)了吗？ / Have you read the [contribution guide](https://github.com/langbot-app/LangBot/blob/master/CONTRIBUTING.md)?
+- [ ] 我已签署或将在机器人提示后签署 [CLA](https://github.com/langbot-app/LangBot/blob/master/CLA.md)。 / I have signed, or will sign when prompted by the bot, the [CLA](https://github.com/langbot-app/LangBot/blob/master/CLA.md).
 - [ ] 与项目所有者沟通过了吗？ / Have you communicated with the project maintainer?
 - [ ] 我确定已自行测试所作的更改，确保功能符合预期。 / I have tested the changes and ensured they work as expected.
 

.github/workflows/cla.yml

@@ -0,0 +1,41 @@
+name: "CLA Assistant"
+on:
+  issue_comment:
+    types: [created]
+  pull_request_target:
+    types: [opened, closed, synchronize]
+
+permissions:
+  actions: write          # re-run the failed CLA check after signing
+  contents: read          # signatures are stored in the remote langbot-app/cla repo
+  pull-requests: write    # post guidance comments, lock PR after merge
+  statuses: write         # set the commit status
+
+jobs:
+  CLAAssistant:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "CLA Assistant"
+        if: (github.event.comment.body == 'recheck' || github.event.comment.body == 'I have read the CLA Document and I hereby sign the CLA') || github.event_name == 'pull_request_target'
+        # Upstream repo was archived in 2026-03; pin to the v2.6.1 commit SHA.
+        uses: contributor-assistant/github-action@ca4a40a7d1004f18d9960b404b97e5f30a505a08 # v2.6.1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # repo-scope PAT with write access to langbot-app/cla
+          PERSONAL_ACCESS_TOKEN: ${{ secrets.CLA_PAT }}
+        with:
+          path-to-document: 'https://github.com/langbot-app/LangBot/blob/master/CLA.md'
+          remote-organization-name: 'langbot-app'
+          remote-repository-name: 'cla'
+          path-to-signatures: 'signatures/version1/cla.json'
+          branch: 'main'
+          allowlist: 'dependabot[bot],github-actions[bot],devin-ai-integration[bot],Copilot,renovate[bot],bot*'
+          custom-notsigned-prcomment: |
+            Thank you for your contribution! :heart: Before we can merge this pull request, we need you to sign the [LangBot Contributor License Agreement (CLA)](https://github.com/langbot-app/LangBot/blob/master/CLA.md). You keep full copyright of your code — the CLA grants us a license to use and distribute your contribution. Signing takes 10 seconds and covers all repositories in this organization, permanently.
+
+            感谢您的贡献！合并前请阅读并签署[贡献者许可协议（CLA）](https://github.com/langbot-app/LangBot/blob/master/CLA.md)。您保留代码的全部版权，签署仅需回复下方指定内容，一次签署对本组织全部仓库永久有效。
+          custom-allsigned-prcomment: 'All contributors have signed the CLA. :white_check_mark: 所有贡献者均已签署 CLA。'
+          lock-pullrequest-aftermerge: true
+        # SECURITY: this workflow runs on pull_request_target (it holds secrets and has
+        # write access to the base repository). NEVER add an actions/checkout step that
+        # checks out the PR's code here.