ci: run only Checkstyle in the merge queue; make AMD/Intel manual-only (#1515)

## Summary

The AMD and Intel GPU runners are currently unavailable, so those CI
jobs fail whenever they run — including inside the **merge queue**,
which blocks all merges for some time. This PR makes the merge queue run
**only Checkstyle** for now.

## Change

Reduce the **AMD GPU** and **Intel GPU** workflow triggers to
`workflow_dispatch` only, mirroring the **NVIDIA GPU** workflow's `on:`
(manual trigger) **minus the schedule** — we don't maintain a schedule
for AMD/Intel.

This removes their `push`, `pull_request`, and `merge_group` triggers,
so:

- **Only Checkstyle runs in the merge queue** (it keeps its
`merge_group` trigger).
- AMD/Intel no longer run automatically on PRs or pushes.
- AMD/Intel can still be launched on demand via the Actions **"Run
workflow"** button once the runners are healthy again.

`checkstyle.yml` is unchanged.

## Notes

- Observed (not changed here): `merge-queue-failure.yml` listens for the
**NVIDIA GPU** workflow with `event == 'merge_group'`, but NVIDIA GPU
has no `merge_group` trigger, so that handler is currently a no-op. Can
be addressed separately.

Co-authored-by: Copilot <[email protected]>

.github/workflows/amd-ci.yml

@@ -1,27 +1,10 @@
 name: AMD GPU
 
+# AMD runners are currently unavailable, so this suite is manual-only
+# (mirrors NVIDIA GPU, but without a schedule). Removing the push / pull_request /
+# merge_group triggers keeps it out of PR checks and the merge queue until the
+# runners are healthy again; only Checkstyle runs in the merge queue for now.
 on:
-  push:
-    branches:
-      - main
-    paths:
-      - "src/**"
-      - "test/**"
-      - "setup.py"
-      - "pyproject.toml"
-      - ".github/workflows/amd-ci.yml"
-  pull_request:
-    branches:
-      - main
-    paths:
-      - "src/**"
-      - "test/**"
-      - "setup.py"
-      - "pyproject.toml"
-      - ".github/workflows/amd-ci.yml"
-  merge_group:
-    branches:
-      - main
   workflow_dispatch:  # Enables manual trigger
 
 concurrency:

.github/workflows/intel-ci.yml

@@ -1,21 +1,10 @@
 name: Intel GPU
 
+# Intel runners are currently unavailable, so this suite is manual-only
+# (mirrors NVIDIA GPU, but without a schedule). Removing the push / pull_request /
+# merge_group triggers keeps it out of PR checks and the merge queue until the
+# runners are healthy again; only Checkstyle runs in the merge queue for now.
 on:
-  push:
-    branches:
-      - main
-    paths:
-      - "src/**"
-      - "test/**"
-  pull_request:
-    branches:
-      - main
-    paths:
-      - "src/**"
-      - "test/**"
-  merge_group:
-    branches:
-      - main
   workflow_dispatch:  # Enables manual trigger
 
 concurrency: