refactor(toolchain): relocate setup step 6 (topic-docs.yaml) to the plugin that owns it

[kyle-sexton]

Step 6 writes another plugin's consumer config (implementation/verification topic-docs binding) — move to a verification or shared lifecycle setup skill.

Origin: skill-name audit + interview, contract at docs/topics/shadowed-skill-renames/PLAN.md (PR #256).

[kyle-sexton]

This was generated by AI during triage (historic normalization, rule 8g).

Sites + fix pattern: plugins/toolchain/skills/setup/SKILL.md (remove Step 6), plugins/toolchain/reference/topic-docs.md (move ownership note), plugins/toolchain/README.md + plugins/toolchain/CHANGELOG.md (record the move). Destination plugin (verification or a shared lifecycle-setup skill) needs picking at implementation time — mechanical relocation either way, no design fork blocks starting.