Let the git remote be set from Settings

Attaching a workspace to an existing repository meant leaving for a
terminal: gitsync could init, commit and push, and could tell whether an
'origin' existed, but had no way to create one, and nothing in the
settings held a remote URL. Sync just logged "no 'origin' remote
configured" and committed locally.

Settings > Git remote... now shows the workspace repository's origin and
lets it be set, repointed or cleared. The URL is read from and written
to the repository, never to config.json, because a remote belongs to the
checkout — so each workspace shows its own, and the box is refreshed
whenever the repository in play changes.

This is separate from the Gitea publishing in New project, which is a
cookiecutter post-gen hook and only ever applies to generated projects.

Also factors out first_output_line, which repo_root and the new
origin_url were both open-coding, and origin_args, so the add-vs-set-url
decision can be tested without a repository on disk.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBWj9TphFMCoh7VHaSRnvQ

README.md

@@ -63,14 +63,21 @@ Terminal=false
    or click **📂** to pick a folder in a native file browser — and press **Open**.
    The directory is created if it does not exist.
 2. Press **Init git** once to make the workspace a git repository. To sync with
-   another machine, add a remote yourself:
+   another machine, point it at a remote through **Settings ▸ Git remote…**:
+   paste the repository's clone URL into the **origin** box and press **Set
+   origin**. The repository has to exist on the server already — this attaches
+   to it, it does not create it. Clearing the box and pressing Set detaches
+   again. The equivalent from a terminal, if you prefer:
    ```sh
    cd ~/Manuscript
    git remote add origin <url>
-   git push -u origin main
    ```
+   Either way the URL lives in the repository's own `.git/config`, not in this
+   app's settings, so every workspace keeps its own.
+
    After that, the **⟳ Sync (git)** button commits all changes, `pull --rebase`s,
-   and pushes.
+   and pushes (setting the upstream on the first push). Until origin is set it
+   commits locally and says so in the log.
 
    If you open a folder that is *not* itself a repository but sits inside one
    (a parent directory is a git work tree), the app asks whether to **use the
@@ -509,6 +516,14 @@ push command alone and is not written into the project's `.git/config`.
 The token is stored in `config.json` in plain text, the same as the Mistral API
 key.
 
+Note that this is the *only* place the app creates a remote repository, and it
+runs entirely inside the template's hook — the app itself never talks to Gitea.
+So it applies to projects made through **New project** and nothing else, and it
+does nothing if **Run the template's hooks** is off, if any of the three Gitea
+settings is blank, or if the template has no such hook. For any workspace that
+did not come from the template, attach the remote with **Settings ▸ Git
+remote…** instead.
+
 ## New files from a template
 
 The file list has two create buttons: