Reduce sync-issues workflow frequency to daily schedule only

[c-vigo]

Description

The sync-issues.yml workflow currently triggers on multiple events:

• Schedule: daily at midnight UTC (0 0 * * *)
• Issues: opened, reopened, closed
• Pull requests: opened, closed
• Manual: workflow_dispatch

The event-driven triggers (issues and pull_request) cause the workflow to run on every issue or PR state change, which clutters the Actions tab and generates unnecessary commits and workflow runs.

Proposed Change

Remove the issues and pull_request event triggers, keeping only:

• Schedule: daily at night (e.g. 0 2 * * * — 2:00 AM UTC)
• Manual: workflow_dispatch (for on-demand syncs when needed)

This also allows simplifying the target branch decision logic. Currently the workflow handles a pull_request merged-into-main case that becomes dead code once the PR trigger is removed. The simplified logic would be:

• workflow_dispatch with target-branch input → use that value (validated against allowed branches)
• Everything else (schedule, dispatch without input) → dev

Note: The workflow_dispatch trigger with target-branch input must be preserved — it is used by release.yml (targets release/$VERSION) and post-release.yml (targets dev) to trigger on-demand syncs during the release cycle.

Acceptance Criteria

• Remove issues and pull_request triggers from sync-issues.yml
• Keep schedule and workflow_dispatch triggers
• Simplify target branch logic (remove PR-merged-into-main branch)
• Optionally adjust cron time if needed
• Verify release.yml and post-release.yml dispatch calls still work (no changes needed there)