Extract github-workflows (actions + reusable workflows) + personal-stack adopt

[jorisjonkers-dev-agents]

Objective: versioned reusable composite actions + workflows; consumed via pinned uses:.

Notes: parameterize path/host/image/script/package-manager deltas as inputs; setup-node must support pnpm + Yarn Berry; setup-java-gradle accepts GitHub Packages creds.

Work:

• Move prepare-ci-host / setup-java-gradle / setup-node-* + reusable fragments
• personal-stack consumes pinned refs
• Pipeline Complete pipeline built from shared reusable workflows

Part of #605.

[jorisjonkers-dev-agents]

Extraction merged. ExtraToast/github-workflows#1 (Codex-high; CI green: actionlint + yaml). actions/{prepare-ci-host,setup-java-gradle,setup-node[pnpm|yarn]} + reusable jvm-ci.yml. REMAINING: release v0.1.0, then personal-stack consumes pinned uses: refs (and Renovate keeps them current).