Merge pull request 'agentws: check out --branch from origin when it exists' (#17) from benvin/agentws-branch-checkout into main

Reviewed-on: #17

README.md

@@ -109,10 +109,10 @@ checkout too; the worktrees themselves live under the **worktree root**
 
 ```bash
 # Clone unkin/argocd-apps into ~/src/prodenv if missing, then add a worktree for
-# a new branch off the remote default branch. Prints the worktree path.
+# the branch. Prints the worktree path.
 agentws new argocd-apps --branch benvin/my-change
 
-# Branch off a specific base instead of the remote default
+# Branch off a specific base instead of the remote default (new branches only)
 agentws new argocd-apps --branch benvin/hotfix --from release-1.2
 
 # List managed worktrees (repo, branch, path)
@@ -136,6 +136,20 @@ agentws clean
 agentws token
 ```
 
+`agentws new` fetches first, then takes one of two paths and names the one it
+took on its last output line. A branch that **already exists on origin** is
+checked out at `origin/<branch>` and set to track it, so the worktree starts on
+the branch's own commits (`branch <b> (tracking origin/<b> at <sha>)`); a local
+branch left from an earlier run is fast-forwarded onto it. A branch origin does
+**not** have is created from `--from`, or from the remote's default branch when
+`--from` is absent (`branch <b> (new, from origin/<base>)`) — the default is read
+from `origin/HEAD`, so a repo on `master` forks from `master`. `--from` is
+ignored, with a note, when the branch is already on origin.
+
+The one case the worktree does not land on `origin/<branch>` is a local branch
+carrying commits origin has never seen. Those commits exist nowhere else, so the
+checkout is left on them and the output says how many.
+
 ### prune
 
 `agentws prune` finds worktrees two ways and merges the results: the managed