ci: add cancel-in-progress concurrency to PR helper workflows

[rusackas]

SUMMARY

pr-lint, the GitHub Actions validator (github-action-validator), and the
labeler all re-run on every push to a PR (synchronize) but had no
concurrency group, so rapid successive pushes left superseded runs going to
completion instead of being cancelled.

Add the standard PR-keyed concurrency block already used across the other
workflows:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }}
  cancel-in-progress: true

These are idempotent / last-run-wins workflows (title lint, action validation,
label sync), so cancelling an in-flight run is safe — the newest push re-runs
them. Small per-PR savings, but free.

Left intentionally untouched: welcome-new-users (one-shot on PR open),
showtime-trigger (label/unlabel semantics where cancellation could drop a
sync), and issue_creation (not synchronize-driven).

TESTING INSTRUCTIONS

Push twice in quick succession to a PR and confirm the first run of each of
these workflows is cancelled in favor of the latest.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

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