Make "Create PR" available in all repos (non-GitHub remotes & local), not only GitHub-connected ones

### What problem are you trying to solve?

Today the **"Create PR"** action is only available for repositories connected to GitHub. Developers frequently work in repos that aren't (or can't be) GitHub-connected:

- **Non-GitHub remotes** — GitLab, Bitbucket, Azure DevOps, self-hosted/enterprise git
- **Local-only repos** — no remote configured yet
- **Air-gapped / private mirrors**

In these cases the agent produces changes on a branch, but there's no one-click way to open a PR/MR (or push + surface the compare link), forcing a manual switch to the terminal or the provider's web UI. This breaks the "idea → pull request" flow the app is built around.

### Proposed solution

Make **"Create PR" available in all repos**, adapting to the underlying git host:

- **GitHub remote** → open a PR (current behavior).
- **Other remotes (GitLab/Bitbucket/ADO/etc.)** → push the branch and open the provider's "create MR/PR" compare URL, or create it via the provider API when a connection exists.
- **Local-only repo** → push to (or help configure) a remote, then surface the compare link; at minimum, offer "push branch & copy compare URL".

Detect the remote type from the git origin and label the button accordingly (Create PR / Create MR / Push & open compare).

### Workflow impact

Anyone using the app with non-GitHub or local repos gets the same seamless land-your-work experience instead of dropping to the CLI. It broadens the app's reach beyond GitHub-hosted work and keeps the agent flow unbroken regardless of where the code lives.

### Additional context

Related but narrower existing issues: #529 (fork → upstream PRs) and #1246 (connect to Azure DevOps repos). This request is broader: a universal "Create PR" entry point across GitHub-connected, other remotes, and local repositories.

Issue không nêu tên tệp, bài kiểm thử hoặc entry point nào. Hãy bắt đầu bằng cách tìm action Create PR hiện có chỉ dành cho GitHub và lần theo việc phát hiện remote cũng như cách xử lý push branch hoặc liên kết so sánh; công việc được xem là hoàn tất khi có các luồng tương đương được gắn nhãn cho GitHub, các remote khác và các repository chỉ cục bộ.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.