chore(productivity): move writing evals to repo root

[christso]

Summary

Move the public writing eval suite to evals/productivity/write-naturally so evaluation tooling lives outside the installable productivity plugin.

Update the Bun commands, workspace skill path, baseline test import, documentation links and output ignore rule. Move the skill-specific README into plugins/productivity/skills/write-naturally/README.md and update its relative links. The skill instructions, prompts, fixtures and grading rules are unchanged.

Verification

• bun run validate passed.
• bun test: 21 passed.
• Public and generated baseline configurations validated.
• A temporary fixture copied all five skill/documentation files correctly and cleaned the workspace.
• Independent review found no blocking issues.

Model evals were not rerun for this relocation.