fix(style-agent): restore missing 0.6.0 changelog section

PR #111 replaced the `## [0.6.0] - 2026-07-20` heading with `## [0.7.0]`
and left 0.6.0's body orphaned beneath the new section. The result was a
duplicate `### Added` heading inside 0.7.0, the /css skill (all of #109)
filed under the wrong release, and no 0.6.0 entry at all in a file that
ships to users via the marketplace.

Restores the heading above the orphaned block. No content rewritten and
no version bump — plugin.json stays at 0.7.0, which is correct.

Also adds the PR #111 team review artifact under docs/artifacts/, which
is where this defect was found. Note docs/artifacts/index.html is
hand-maintained and does not yet link it.

tests/run.sh green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: shawnsandy