ci(stale): bump operations-per-run and add manual trigger

The stale workflow was hitting its 250 operations-per-run cap every
daily run, causing the "No more operations left! Exiting..." warning
and leaving a growing backlog of stale-labeled items that were never
being closed. GitHub API headroom is plentiful (250 ops uses ~1.6% of
the 15k/hour bucket), so raising to 1500 drains the backlog without
any rate-limit risk.

Also adds workflow_dispatch so maintainers can trigger the workflow
from the Actions tab or via gh workflow run stale.yml.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/stale.yml

@@ -2,6 +2,7 @@ name: 'Handle stale issues and PRs'
 on:
   schedule:
     - cron: '30 1 * * *'
+  workflow_dispatch:
 
 jobs:
   stale:
@@ -9,7 +10,7 @@ jobs:
     steps:
       - uses: actions/stale@v10
         with:
-          operations-per-run: 250
+          operations-per-run: 1500
           days-before-stale: 90
           days-before-close: 30
           stale-issue-label: 'stale'