Cancel superseded PR CI runs

[ChrisRackauckas]

🚧 UNREVIEWED — awaiting review by @ChrisRackauckas. Filed by an AI agent running as @ChrisRackauckas; Chris has not seen this.
Harness: Claude Code 2.1.302 · Model: claude-sonnet-5-5
Conversation: local subagent of the fleet-master session (no published session URL)

This PR should be ignored until reviewed by @ChrisRackauckas.

Adds a top-level concurrency group with cancel-in-progress to the pull_request workflows that lack one, so a new push to a PR cancels that PR's superseded run. cancel-in-progress is true only for pull_request events, so pushes to the default branch, tags and scheduled runs never cancel each other.

Workflows changed:

• DependabotAutoMerge.yml
• DocPreviewCleanup.yml
• RunicSuggestions.yml
• SpellCheck.yml
• benchmark.yml

Workflows skipped:

• none

Verification: every edited file parses as YAML and differs from the original only by the concurrency key (checked programmatically). CI itself is the runtime check; I have not run the workflows locally.

Risk assessment

• Risk: low (L3: CI configuration)
• Blast radius: PR CI runs only; no source, test or public API change. Pushes to the default branch, tags and schedules are unaffected by the conditional.
• Evidence: YAML round-trip check; an equivalent group with a startsWith(github.ref, 'refs/pull/') condition is already used by the existing CI workflows in these repos.
• Independent review: Codex gpt-6.1-sol rated it low (L3), high confidence: mechanical, PR-only CI configuration change with valid YAML and no group collision with the reusable workflows. It noted that a queued (not running) default-branch run can be replaced by a newer push to the same group, which cancel-in-progress: false does not prevent; that is the same behaviour as the existing startsWith(github.ref, 'refs/pull/') pattern.
• Merge: auto-merge candidate after an independent review

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🤖 Posted by an AI agent — harness: Claude Code 2.1.302 · model: claude-sonnet-5-5
Conversation: local subagent of the fleet-master session (no published session URL)