chore(release): promote changelog into [4.0.6]

[bpamiri]

Summary

Playbook pre-flight step 2 for 4.0.6: promote changelog fragments on develop first so [Unreleased] does not go stale and force a back-port after the cut.

Ran:

tools/changelog-promote.sh --preview
tools/changelog-promote.sh 4.0.6 2026-08-20

--preview confirmed [Unreleased] was already empty (marker only) and every note comes from fragments. The script emits the single-hash # [version](tag-url) => date heading that release.yml expects.

Promoted 48 changelog.d/ fragments (79 entries) into:

# [4.0.6](https://github.com/wheels-dev/wheels/releases/tag/v4.0.6) => 2026-08-20

Verified after the write:

• [Unreleased] is empty (do-not-edit marker only).
• Separator immediately below the new section is exactly --- (three dashes), not ----.
• Simulated awk '/^# \[4.0.6\]/,/^---$/' extracts the 4.0.6 notes with no bleed into 4.0.5.
• Fragment files deleted; changelog.d/README.md kept.
• wheels.json left at 4.0.6 (no version bump in this PR).
• No other files changed.

The release branch is not cut in this PR. This is the develop-side changelog promote only.

Type of Change

• Documentation update

Feature Completeness Checklist

• DCO sign-off -- commit carries Signed-off-by:
• Tests -- n/a (promote-script output only; no runtime code)
• Framework Docs -- n/a
• AI Reference Docs -- n/a
• CLAUDE.md -- n/a
• Changelog fragment -- n/a (this PR is the promotion)
• Test runner -- n/a (no runtime code)

Test Plan

• Confirm [Unreleased] is marker-only
• Confirm the # [4.0.6](...) => 2026-08-20 heading
• Confirm the separator under that section is exactly ---
• Confirm changelog.d/ contains only README.md
• Confirm wheels.json is still "4.0.6"