CircleCI config hardening: enable `--next` validation

# CircleCI config hardening: enable `--next` validation

## Summary

Plan and implement `--next` validation for CircleCI configs so we catch [August 17, 2026 breaking changes](https://discuss.circleci.com/t/breaking-changes-config-compilation-updates-august-17-2026/54719) before they block pipelines.

**Target date:** Before 2026-08-17 (recommend completing by 2026-08-01)

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## Background

On **August 17, 2026**, CircleCI will enforce stricter config compilation. Pipelines using affected patterns will **fail to compile**. The three changes are:

1. **Undeclared parameters** — references to undeclared/out-of-scope pipeline parameters fail instead of resolving to `null`
2. **Regex engine update** — negative lookaheads/lookbehinds, possessive quantifiers, and backreferences are rejected
3. **Config v2.0 sunset** — configs must comply with v2.1 semantics (HEREDOC `<<`, job names, workflow filters, etc.)

Preview affected configs today:

```bash
circleci config validate --next
```

Start by locating this repository's CircleCI configuration files and run `circleci config validate --next` against them. Review any reported undeclared parameters, rejected regular expressions, or v2.0 compatibility issues; the work is done when the affected configurations pass the preview validation before the stated deadline.

Written by the indexing model from the issue text.