Remove all skills and agents from drupal-devkit; remap to owner repos per #3585585

Feature Request

Description

drupal-devkit should not host any skills or agents directly. Per the skill-ownership standard (#3585585), every skill and agent belongs in the repo of the thing it's about (its contrib module, ABP, or a dedicated upstream repo). Devkit's job is to install — not to bundle content.

Goal: empty plugins/drupal-core/skills/ and plugins/drupal-core/agents/ entirely, with each item remapped to its owner repo and reachable through the marketplace.

Plan

• Produce a per-skill remapping table (current location → target owner repo → status of that repo)
• For each contrib-module skill, open an upstream issue / MR on the module's repo proposing the move (e.g. drupal-eca → ECA module, drupal-search-api → Search API, drupal-storybook → Storybook integration)
• For cross-cutting / core skills (e.g. using-drupal-core, drupal-coding-standards, drupal-at-your-fingertips), move into ABP catalog or a dedicated upstream repo
• For each move, add the new home as a marketplace entry in default_marketplaces.json so users still receive it via drupal-devkit install
• Delete the in-tree skill once the upstream landing is confirmed; bump CHANGELOG with the migration path
• Remove the now-empty plugins/drupal-core/skills/ and agents/ directories

Current inventory in plugins/drupal-core/skills/

accessibility, custom-drupal-module, debug, drupal-at-your-fingertips, drupal-coding-standards-rt, drupal-content-moderation, drupal-contrib-mgmt, drupal-contribute-fix, drupal-ddev, drupal-docs-explorer, drupal-eca, drupal-field-system, drupal-frontend-expert, drupal-json-api, drupal-sdc, drupal-search-api, drupal-security-audit, drupal-storybook, drupal-views, drupal-visual-regression, drush, migrate, performance, refactor, using-drupal-core.

Use Case

Aligns devkit with #3585585 (skills owned by the things they're about). Reduces devkit's surface area to "installer code only", makes upgrades to skills independent of devkit releases, and removes the implicit claim that devkit is the canonical source for any given skill's content.