chore: refine Renovate labels and automerge rules

[gargiulofrancesco]

Add changelog/chore automatically so Renovate PRs satisfy the required label check.

Configure auto-merge to:

• Allow initial GitHub Actions commit pinning (pinDigest).
• Require manual merging of Rust toolchain updates.

Keep the inherited Spiral settings and existing required merge checks. Ongoing Actions digest updates remain manual.

Validation: JSON syntax validation and git diff --check.

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 32 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing chore/renovate-labels-automerge (94aab8c) with develop (e668052)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩