fix: use 1-based runner indices for Gitea Actions compatibility

scripts/ci/distribute_molecule.py

@@ -9,7 +9,7 @@ Each pair is printed as ``scenario|platform_name|platform_image|platform_command
 so the CI workflow can set the appropriate environment variables.
 
 Usage:
-  python3 scripts/distribute_molecule.py --runner-index 0 --max-runners 3
+  python3 scripts/distribute_molecule.py --runner-index 1 --max-runners 3
   # prints: default|ubuntu-2204|ubuntu:22.04| lifecycle|ubuntu-2204|ubuntu:22.04| ...
   python3 scripts/distribute_molecule.py --list
   # prints all scenarios, one per line
@@ -105,7 +105,8 @@ def pairs_for_runner(pairs: list[TestPair], runner_index: int, max_runners: int)
     "--runner-index",
     type=int,
     default=None,
-    help="Zero-based runner index. If omitted, prints all groups.",
+    help="One-based runner index (Gitea Actions renders 0 as empty). "
+    "Converted to zero-based internally. If omitted, prints all groups.",
 )
 @click.option(
     "--max-runners",
@@ -143,7 +144,9 @@ def cli(runner_index: int | None, max_runners: int, list_all: bool, list_platfor
             labels = " ".join(p.encode() for p in group) if group else "(none)"
             click.echo(f"Runner {i}: {labels}")
         return
-    assigned = pairs_for_runner(pairs, runner_index, max_runners)
+    # Convert 1-based CLI index to 0-based internal index
+    zero_based = runner_index - 1
+    assigned = pairs_for_runner(pairs, zero_based, max_runners)
     click.echo(" ".join(p.encode() for p in assigned))