Harden and correct keep-alive GHA workflow

[bart-vmware]

Several correctness, reliability and maintainability issues were found and fixed in the keep-alive workflow:

Correctness

• Fix default branch detection: github.event.repository.default_branch is empty on scheduled runs; now resolved via the GitHub API
• Fix $GITHUB_OUTPUT references that were unquoted
• Fix branch strategy: the old approach accumulated commits on the keep-alive-workflow branch across runs; now always resets to the tip of the default branch before force-pushing a single empty commit
• Fix PR strategy: gh pr reopen fails after a force push because GitHub considers the head branch changed; replaced the reopen/close cycle with keeping a single PR permanently open
• Fix jq returning literal null: add // empty so missing values produce an empty string instead
• Remove 2>/dev/null from gh calls, which was silently swallowing errors

Reliability

• No set -euo pipefail was present in any run block, allowing intermediate command failures to be silently swallowed. Added to all three blocks.
• Add explicit git fetch origin before checkout, since actions/checkout with fetch-depth: 1 does not reliably establish remote tracking refs
• Add guard to fail fast if the API returns an empty default branch
• Add numeric validation of the parsed PR number
• Add concurrency group to prevent simultaneous runs
• Add workflow_dispatch bypass to always run keep-alive on manual triggers
• Add a new step that explicitly re-enables all scheduled workflows via the GitHub API (PUT /enable), resetting their individual 60-day inactivity timers as a belt-and-suspenders measure alongside the branch push
• Replace process substitution <(...) with command substitution $(...) in the enable step so set -e correctly propagates API failures

Simplification

• Hoist GH_TOKEN, GH_REPO, and BRANCH_NAME to job-level env to reduce duplication across steps
• Split the two monolithic steps into three focused ones: enable workflows, push branch, open PR
• Replace issues: read (unused) with actions: write (required for API enablement)
• Rename branch from keep-alive-workflow to keep-workflows-alive
• Add build status badge and an important notice to the PR body

[TimHess]

LGTM