ci(checks): let duplicate PR runs finish

[chhoumann]

Summary

Duplicate PR events currently cancel required checks on the same head, leaving the aggregate rollup at FAILURE. Remove concurrency from Test, Docs, PR Title, and Dependency Review because native groups cannot distinguish a duplicate head from a superseded commit and still cancel pending runs with cancel-in-progress: false. Superseded runs may finish; the extra CI work is cheaper than a custom cancellation mechanism.

The baseline probe measured CLEAN merge state after the latest required checks passed, despite the failed aggregate rollup. Cancelled duplicates do not universally block the ruleset.

Verification

• Node 22.23.2: npm run lint, npm run format:check, npm run typecheck, npm run build, npm run test (81 files, 1,263 tests), and npm run docs:build all pass. Actionlint 1.7.12 passes.
• Real draft probe #379 captures the failing baseline and fixed duplicate checks. Check listings show a FAILURE baseline, then SUCCESS on duplicate and superseded fixed heads with zero cancellations. Production pull_request_target still needs a post-merge probe because GitHub reads its workflow from the default branch.
• Obsidian verification was not needed for workflow-only changes. No plugin code changed.

Release impact

None. Release workflows, required check names, permissions, and the ruleset are unchanged.

Note

Remove workflow-level concurrency groups from 4 CI workflows

Removes the concurrency blocks (group + cancel-in-progress) from the Dependency Review, Documentation, PR Title, and Test workflows in .github/workflows/. Duplicate PR runs can now finish instead of cancelling each other. Documents the rationale in AGENTS.md: cancelled runs can make the commit check rollup fail, so concurrency groups should not be added to these workflows.

^{Macroscope summarized fd524f6.}

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