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.

The issue names no files, tests, or entry points. Start by locating the existing GitHub-only Create PR action and tracing remote detection and branch-push or compare-link handling; done means equivalent labeled flows are available for GitHub, other remotes, and local-only repositories.

索引モデルが issue の本文から書いたものです。