Adopt arcade inter-branch merge workflow for main → net10.0 and main → net11.0 (dotnet#34144)

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Adopts the shared `dotnet/arcade` inter-branch merge infrastructure
(used by 20+ dotnet repos including runtime, aspnetcore, sdk, efcore,
msbuild) to replace our custom weekly schedule-based workflow and manual
merge script.

### Changes

- **New** `.github/workflows/merge-main-to-net10.yml` — merges `main` →
`net10.0` using arcade
- **New** `.github/workflows/merge-main-to-net11.yml` — merges `main` →
`net11.0` using arcade
- **New** `github-merge-flow-net10.jsonc` — config for main → net10.0
- **New** `github-merge-flow-net11.jsonc` — config for main → net11.0
- **Removed** `.github/workflows/weekly-branch-merge.yml` — replaced by
above
- **Removed** `.github/scripts/MergeBranchAndCreatePR.ps1` — no longer
needed

### Why two workflows?

The arcade infrastructure supports one `MergeToBranch` per source branch
per config file. To merge `main` into both `net10.0` and `net11.0`
independently (not chained), we use two workflows with separate config
files.

### How it works

1. A PR merges to `main`
2. Both workflows trigger simultaneously
3. Each creates (or updates) a merge PR: `merge/main-to-net10.0` →
`net10.0` and `merge/main-to-net11.0` → `net11.0`
4. The merge PRs wait for human review — nothing is auto-merged
5. `ResetToTargetPaths` auto-resets version files (`global.json`,
`eng/Versions.props`, etc.) to avoid common conflicts

### Triggers

| Trigger | Purpose |
|---|---|
| `push` to `main` | Immediate — runs on every merge to main |
| `schedule` (daily 3 AM UTC) | Safety net — catches missed push events
|
| `workflow_dispatch` | Manual — run on-demand from Actions UI |

### Comparison

| | Old | New |
|---|---|---|
| Trigger | Weekly cron (Mondays) | On push + daily cron + manual |
| Merge topology | Custom script | Arcade shared infrastructure |
| Version file conflicts | Manual resolution | `ResetToTargetPaths`
auto-resets |
| Maestro-only commits | Creates PR anyway | Skipped automatically |
| Notifications | Default @-mentions | `-QuietComments` suppresses noise
|
| Existing PR handling | Fails if PR exists | Updates existing PR in
place |
| Maintenance | Custom MAUI-only script | Centralized in arcade, shared
across 20+ repos |

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: PureWeen <5375137+PureWeen@users.noreply.github.com>
Co-authored-by: Shane Neuville <shneuvil@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.github/scripts/MergeBranchAndCreatePR.ps1

@@ -0,0 +1,32 @@
+# Merge main → net10.0
+# Uses the shared dotnet/arcade merge flow infrastructure.
+# Configuration: /github-merge-flow-net10.jsonc
+#
+# Key features (from arcade):
+#   - Event-driven: triggers on push, with daily cron safety net
+#   - ResetToTargetPaths: auto-resets version files to target branch versions
+#   - QuietComments: reduces GitHub notification noise
+#   - Skips PRs when only Maestro bot commits exist
+#   - Updates existing open PR instead of creating new ones
+
+name: Merge main to net10.0
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+
+  # Daily safety net in case a push event is missed
+  schedule:
+    - cron: '0 3 * * *'
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  Merge:
+    uses: dotnet/arcade/.github/workflows/inter-branch-merge-base.yml@main
+    with:
+      configuration_file_path: 'github-merge-flow-net10.jsonc'

.github/workflows/merge-main-to-net10.yml

@@ -0,0 +1,32 @@
+# Merge main → net11.0
+# Uses the shared dotnet/arcade merge flow infrastructure.
+# Configuration: /github-merge-flow-net11.jsonc
+#
+# Key features (from arcade):
+#   - Event-driven: triggers on push, with daily cron safety net
+#   - ResetToTargetPaths: auto-resets version files to target branch versions
+#   - QuietComments: reduces GitHub notification noise
+#   - Skips PRs when only Maestro bot commits exist
+#   - Updates existing open PR instead of creating new ones
+
+name: Merge main to net11.0
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+
+  # Daily safety net in case a push event is missed
+  schedule:
+    - cron: '0 3 * * *'
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  Merge:
+    uses: dotnet/arcade/.github/workflows/inter-branch-merge-base.yml@main
+    with:
+      configuration_file_path: 'github-merge-flow-net11.jsonc'