Fix claim-after-discard: check branch data instead of branch existence

After discard clears item data from a branch (when DOLT_BRANCH deletion
fails), the branch still exists but is empty. The old forkBranchExists
check saw the branch and used write/{branch}/{branch}, starting from
the empty state — making the PR show the item as removed.

branchHasData queries the specific wanted item on the branch. Dead
branches (cleared by discard) return false, so Exec starts fresh from
main via write/main/{branch}.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: julianknutsen

internal/backend/remote.go

@@ -89,7 +89,7 @@ func (r *RemoteDB) Exec(branch, _ string, _ bool, stmts ...string) error {
 	// If the target branch already exists on the fork, write from that branch
 	// to preserve prior mutations (e.g. claim → done). Otherwise write from main.
 	fromBranch := "main"
-	if branch != "main" && r.forkBranchExists(branch) {
+	if branch != "main" && r.branchHasData(branch) {
 		fromBranch = branch
 	}
 
@@ -386,8 +386,28 @@ func formatDiffRow(buf *strings.Builder, header, fields []string) {
 	}
 }
 
-// forkBranchExists checks whether a branch exists on the fork database.
-func (r *RemoteDB) forkBranchExists(branch string) bool {
+// branchHasData checks whether a wl/ branch has item data worth preserving.
+// Branches cleared by discard (no wanted row) should start fresh from main.
+func (r *RemoteDB) branchHasData(branch string) bool {
+	// Extract wanted ID from wl/{rig}/{wantedID} convention.
+	parts := strings.SplitN(branch, "/", 3)
+	if len(parts) != 3 || parts[0] != "wl" || parts[2] == "" {
+		// Not a wl branch — fall back to branch existence check.
+		return r.branchExists(branch)
+	}
+	wantedID := strings.ReplaceAll(parts[2], "'", "''")
+	sql := fmt.Sprintf("SELECT COUNT(*) AS cnt FROM wanted WHERE id='%s'", wantedID)
+	csv, err := r.queryForkBranch(sql, branch)
+	if err != nil {
+		// Branch probably doesn't exist — start from main.
+		return false
+	}
+	lines := strings.Split(strings.TrimSpace(csv), "\n")
+	return len(lines) >= 2 && strings.TrimSpace(lines[1]) != "0"
+}
+
+// branchExists checks whether a branch exists on the fork database.
+func (r *RemoteDB) branchExists(branch string) bool {
 	escaped := strings.ReplaceAll(branch, "'", "''")
 	sql := fmt.Sprintf("SELECT COUNT(*) AS cnt FROM dolt_branches WHERE name='%s'", escaped)
 	csv, err := r.queryForkBranch(sql, "main")