fix: allow fork overrides for overlay jobs, error only on image/alert flags

Cross-checked with openshift/release step-ref YAMLs. Overlay jobs
support GITHUB_ORG_NAME, GITHUB_REPOSITORY_NAME, and
RELEASE_BRANCH_NAME overrides but NOT IMAGE_REPO, IMAGE_REGISTRY,
TAG_NAME, or SKIP_SEND_ALERT. The script now allows --org, --repo,
--branch for overlay jobs and errors only on the unsupported flags.

Assisted-by: OpenCode

Author: zdrapela

skills/prow-trigger-nightly/SKILL.md

@@ -80,7 +80,7 @@ GKE and OSD-GCP each share a single cluster — never run two jobs on the same p
 
 ## Step 2: Options
 
-**Important:** Overlay repo jobs do NOT support parameter overrides via Gangway. The script will error if override flags are passed for an overlay job. If the user selected an overlay job, skip this step and go directly to Step 3. Mention that overlay jobs always run with their default configuration.
+**Important:** Overlay repo jobs only support fork overrides (`--org`, `--repo`, `--branch`). Image overrides (`--image-registry`, `--image-repo`, `--tag`) and `--send-alerts` are NOT supported — the script will error if these are passed for an overlay job. If the user doesn't need fork overrides, skip this step and go directly to Step 3.
 
 For RHDH repo jobs, present all options together. The user picks by number — multiple selections allowed (e.g. "2, 5"):
 

skills/prow-trigger-nightly/scripts/trigger_nightly_job.py

@@ -293,20 +293,15 @@ def build_payload(args: argparse.Namespace) -> dict:
     is_overlay = args.job.startswith(OVERLAY_JOB_PREFIX)
 
     if is_overlay:
-        # Overlay jobs don't support parameter overrides or alerts.
+        # Overlay jobs support fork overrides only (org, repo, branch).
+        # Image overrides and alerts are not supported.
         unsupported: list[str] = []
         if args.image_repo:
             unsupported.append("--image-repo")
         if args.image_registry:
             unsupported.append("--image-registry")
         if args.tag:
             unsupported.append("--tag")
-        if args.org:
-            unsupported.append("--org")
-        if args.repo:
-            unsupported.append("--repo")
-        if args.branch:
-            unsupported.append("--branch")
         if args.send_alerts:
             unsupported.append("--send-alerts")
         if unsupported:
@@ -315,6 +310,13 @@ def build_payload(args: argparse.Namespace) -> dict:
                 "These flags only work with rhdh repo jobs."
             )
             sys.exit(1)
+
+        if args.org:
+            envs["MULTISTAGE_PARAM_OVERRIDE_GITHUB_ORG_NAME"] = args.org
+        if args.repo:
+            envs["MULTISTAGE_PARAM_OVERRIDE_GITHUB_REPOSITORY_NAME"] = args.repo
+        if args.branch:
+            envs["MULTISTAGE_PARAM_OVERRIDE_RELEASE_BRANCH_NAME"] = args.branch
     else:
         # RHDH repo jobs support full overrides.
         if args.image_repo: