agentws: check out --branch from origin when it exists

`agentws new --branch` always forked from the base branch, so a worktree for a
branch that already existed on origin started at the base's HEAD with none of
the branch's commits, and callers had to reset --hard afterwards to recover.

- Check out an existing origin/<branch> and set the branch to track it
- Fast-forward a stale local branch onto origin, keeping unpushed commits
- Fork from --from or the remote default only when origin lacks the branch
- Name which of the two paths was taken in the command's output
- Ignore --from, with a note, when the branch is already on origin

cmd/agentws/main.go

@@ -161,13 +161,24 @@ func newNewCmd() *cobra.Command {
 				return err
 			}
 
-			// c. Base branch: --from or the remote default.
+			// c. A branch origin already has is work in progress and must be
+			//    checked out as it stands; only a branch origin does not have is
+			//    forked from a base.
+			onRemote := agent.GitRemoteBranchExists(srcDir, "origin", branch)
+			startPoint := "origin/" + branch
 			base := from
-			if base == "" {
-				base, err = agent.GitRemoteDefaultBranch(srcDir, "origin")
-				if err != nil {
-					return err
+			if onRemote {
+				if from != "" {
+					_, _ = fmt.Fprintf(cmd.OutOrStdout(), "note: --from %s ignored, origin/%s already exists\n", from, branch)
 				}
+			} else {
+				if base == "" {
+					base, err = agent.GitRemoteDefaultBranch(srcDir, "origin")
+					if err != nil {
+						return err
+					}
+				}
+				startPoint = "origin/" + base
 			}
 
 			// d. Create the worktree FROM the source checkout so the branch is
@@ -176,7 +187,7 @@ func newNewCmd() *cobra.Command {
 			if _, statErr := os.Stat(wtPath); statErr == nil {
 				return fmt.Errorf("worktree already exists at %s", wtPath)
 			}
-			if err := agent.GitWorktreeAdd(srcDir, wtPath, branch, "origin/"+base); err != nil {
+			if err := agent.GitWorktreeAdd(srcDir, wtPath, branch, startPoint); err != nil {
 				return err
 			}
 
@@ -199,9 +210,19 @@ func newNewCmd() *cobra.Command {
 				return err
 			}
 
-			// f. Report the worktree path and branch.
+			// f. A local branch left from an earlier run may sit behind origin,
+			//    so reusing it is not enough on its own.
+			summary := fmt.Sprintf("branch %s (new, from origin/%s)", branch, base)
+			if onRemote {
+				summary, err = alignToRemote(cmd.OutOrStdout(), wtPath, branch)
+				if err != nil {
+					return err
+				}
+			}
+
+			// g. Report the worktree path and which of the two paths was taken.
 			_, _ = fmt.Fprintf(cmd.OutOrStdout(), "%s\n", wtPath)
-			_, _ = fmt.Fprintf(cmd.OutOrStdout(), "branch %s (from origin/%s)\n", branch, base)
+			_, _ = fmt.Fprintf(cmd.OutOrStdout(), "%s\n", summary)
 			return nil
 		},
 	}
@@ -212,6 +233,47 @@ func newNewCmd() *cobra.Command {
 	return cmd
 }
 
+// alignToRemote puts the worktree on origin/<branch> and reports what that took.
+// A reused local branch can be stale, and a fast-forward is the only move that
+// adds no commit and drops none; a local branch carrying commits origin does not
+// have is left where it stands, because those commits exist nowhere else.
+func alignToRemote(out io.Writer, wtPath, branch string) (string, error) {
+	remoteRef := "origin/" + branch
+	want, err := agent.GitRevParse(wtPath, remoteRef)
+	if err != nil {
+		return "", err
+	}
+	head, err := agent.GitRevParse(wtPath, "HEAD")
+	if err != nil {
+		return "", err
+	}
+	if head != want {
+		ahead, err := agent.GitAheadCount(wtPath, remoteRef, "HEAD")
+		if err != nil {
+			return "", err
+		}
+		if ahead > 0 {
+			return fmt.Sprintf("branch %s (local, %s not on %s, left at %s)",
+				branch, commitCount(ahead), remoteRef, shortSHA(head)), nil
+		}
+		if err := agent.GitMergeFFOnly(wtPath, remoteRef); err != nil {
+			return "", err
+		}
+		_, _ = fmt.Fprintf(out, "fast-forwarded stale %s to %s\n", branch, remoteRef)
+	}
+	if err := agent.GitSetUpstream(wtPath, branch, remoteRef); err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("branch %s (tracking %s at %s)", branch, remoteRef, shortSHA(want)), nil
+}
+
+func shortSHA(sha string) string {
+	if len(sha) > 7 {
+		return sha[:7]
+	}
+	return sha
+}
+
 // --- list -----------------------------------------------------------------
 
 func newListCmd() *cobra.Command {