docs(iterate): close the sloptimizer human-facing adapter backlog item

[easel]

Summary

• The improvement backlog listed an open P3 item claiming the Sloptimizer HELIX adapter still hard-codes the artifact-signal voice profile, blocking prose rewrites for human-facing deliverables.
• That gap was already closed upstream, before this backlog entry was even written: easel-skills' skills/sloptimizer/references/adapters-helix.md has ties its title/rewrite rules to workflows/voice.yml's human-facing profile by name (commits eb13ce6, f5de158, both predating this backlog entry).
• Marks the item done with the real evidence and tracker refs, and updates "Selection for Next Iteration" to reflect the closure and name the next candidate (the P2 healthy-set align brief).
• Regenerated the published mirror (docs/website/content/artifacts/improvement-backlog.md) via uv run scripts/publish-artifacts.py so the site copy matches.

Reviewer notes

• This is a backlog/documentation correction only — no runtime, schema, or gate behavior changes.
• A companion update to easel-skills' adapter doc (bringing it in line with the newly-merged, much-expanded human-facing profile — the claim-vs-label heading distinction, render-doc.js, the fuller avoid list) was made in that separate repo and is out of scope for this PR.

Test plan

• uv run scripts/publish-artifacts.py regenerated the mirror with no drift beyond the intended edit (git diff confirms the two files match)

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