Migrate PR triage workflows to shared workflows

Replace prauto.yml and pr-help-wanted.yml with a single
triage-pull-requests.yml that calls shared reusable workflows from
desktop/gh-cli-and-desktop-shared-workflows:

- triage-label-external-pr: labels external PRs with external,needs-triage
- triage-close-from-default-branch: closes PRs opened from trunk
- triage-pr-requirements: enforces body length + help-wanted issue linkage
- triage-close-no-help-wanted: closes PRs labeled no-help-wanted-issue
- triage-ready-for-review: removes needs-triage on ready-for-review label

Also adds a daily schedule to auto-close PRs with unmet requirements
after 7 days.

Deletes:
- prauto.yml
- pr-help-wanted.yml
- scripts/check-help-wanted.sh

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: BagToad

.github/workflows/pr-help-wanted.yml

@@ -0,0 +1,59 @@
+name: PR Triaging
+on:
+  pull_request_target:
+    types: [opened, reopened, edited, labeled, ready_for_review]
+  schedule:
+    - cron: '0 4 * * *' # Daily at 4 AM UTC — close unmet-requirements PRs
+
+jobs:
+  label-external:
+    if: >-
+      github.event_name == 'pull_request_target' &&
+      (github.event.action == 'opened' || github.event.action == 'reopened')
+    uses: desktop/gh-cli-and-desktop-shared-workflows/.github/workflows/triage-label-external-pr.yml@main
+    permissions:
+      issues: write
+      pull-requests: write
+      repository-projects: read
+
+  close-from-default-branch:
+    if: >-
+      github.event_name == 'pull_request_target' &&
+      github.event.action == 'opened'
+    uses: desktop/gh-cli-and-desktop-shared-workflows/.github/workflows/triage-close-from-default-branch.yml@main
+    with:
+      default_branch: trunk
+    permissions:
+      pull-requests: write
+
+  check-requirements:
+    if: >-
+      github.event_name == 'pull_request_target' &&
+      (github.event.action == 'opened' || github.event.action == 'reopened' || github.event.action == 'edited')
+    uses: desktop/gh-cli-and-desktop-shared-workflows/.github/workflows/triage-pr-requirements.yml@main
+    permissions:
+      issues: read
+      pull-requests: write
+
+  close-unmet-requirements:
+    if: github.event_name == 'schedule'
+    uses: desktop/gh-cli-and-desktop-shared-workflows/.github/workflows/triage-pr-requirements.yml@main
+    permissions:
+      issues: read
+      pull-requests: write
+
+  close-no-help-wanted:
+    if: >-
+      github.event_name == 'pull_request_target' &&
+      github.event.action == 'labeled'
+    uses: desktop/gh-cli-and-desktop-shared-workflows/.github/workflows/triage-close-no-help-wanted.yml@main
+    permissions:
+      pull-requests: write
+
+  ready-for-review:
+    if: >-
+      github.event_name == 'pull_request_target' &&
+      github.event.action == 'labeled'
+    uses: desktop/gh-cli-and-desktop-shared-workflows/.github/workflows/triage-ready-for-review.yml@main
+    permissions:
+      pull-requests: write