Align existing plugin skills with the thin skill authoring contract #534

Summary

Apply the shared plugin skill authoring guidance consistently to the existing PSModule plugin skills.

Findings

• psmodule-pester-migration is a long migration tutorial without an early shared-guide pointer, explicit stop-condition section, thin agent-specific contract, or canonical MSX references.
• psmodule-v8-upgrade lacks the shared-guide pointer and explicit stop conditions, duplicates common migration procedure, and embeds contribution/co-author requirements.
• psmodule-zensical-migration lacks the shared-guide pointer and explicit stop conditions and remains a long common migration procedure.
• psmodule-module-development-orchestration follows the pattern but still contains generic branch/validation detail and a co-author instruction that should be reviewed under the thin-contract rule.
• psmodule-skill-authoring points to the guide and has a stop condition, but lacks a dedicated References section.

Scope

Refactor each skill to keep only agent-specific triggers, decisions, stop conditions, and report-back behavior; link shared procedure to the authoring guide, canonical Process-PSModule documentation, and MSX guidance. Preserve migration intent and validation requirements while avoiding duplicated workflow instructions.

This is follow-up work to #533 and should not expand that draft PR unless explicitly re-scoped.