PLA-322: migrate eslint-plugin-ghost to pnpm

[9larsons]

Refs PLA-322

Summary

• replace Yarn Classic with exact-pinned pnpm 11.19.0 and a frozen pnpm lockfile
• enforce a 72-hour minimum release age, strict dependency builds, and exotic-subdependency blocking with no approved build scripts
• preserve eslint-plugin-self compatibility through pnpm hoisted linking
• migrate developer and release commands to pnpm and pinned @tryghost/pro-ship
• replace the Yarn-only shared lint workflow with an equivalent pinned pnpm Node 20/22 matrix

Validation

• fresh pnpm install --frozen-lockfile after removing yarn.lock
• pnpm lint
• pnpm test (19 passing)
• pnpm pack --dry-run
• pnpm publish --dry-run --no-git-checks
• actionlint .github/workflows/lint.yml

Notes

The existing dependency graph contains TypeScript 7.0.2 while @typescript-eslint 8.65.0 currently declares support below TypeScript 6.1. This version was already locked by Yarn, is preserved by the migration, and the full lint/test suite passes.

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