feat: open an issue when scheduled integration validation fails

[cmarko89]

Why

A new hassfest rule (exact pins on packages Home Assistant depends on are rejected) broke homeassistant-playstationnetwork on an unchanged main. Scheduled runs exist to catch exactly this, but a failed scheduled run notifies nobody, so it only surfaced when an unrelated PR failed.

What

• New alert job: runs only when github.event_name == 'schedule'.
  • When hassfest or HACS fails, it opens a Scheduled validation failing issue (label bug), or comments on the open one.
  • When both pass again, it closes that issue.
  • Push and PR runs never touch the issue.
• The header's caller example now shows a daily schedule and the issues: write grant.

Merge order

This is a draft until all five callers grant issues: write. Without the grant, GitHub rejects the whole called workflow. The caller PRs are:

• ci: validate daily and grant the scheduled-failure alert homeassistant-playstationnetwork#38
• ci: validate daily and grant the scheduled-failure alert homeassistant-pitboss#42
• ci: validate daily and grant the scheduled-failure alert homeassistant-hubspace#31
• ci: validate daily and grant the scheduled-failure alert homeassistant-franklinwh#26
• ci: validate daily and grant the scheduled-failure alert homeassistant-meater#39

Verification

• Branch logic stub-tested: failure with no open issue creates one, failure with an open issue comments, green with an open issue closes it, and green with none does nothing (including a skipped job).
• The issue lookup query was run against the real API.