ci: move workflow to Node 24 actions

[dajiaohuang]

Outcome

Move D&D CI to the current Node 24-based action releases:

• actions/checkout@v4 to actions/checkout@v7
• actions/setup-python@v5 to actions/setup-python@v7
• actions/setup-node@v4 to actions/setup-node@v7
• astral-sh/setup-uv@v6 to immutable astral-sh/setup-uv@v10.0.1

setup-uv v9 changed cache pruning from enabled to disabled by default, so prune-cache: true is explicit here to preserve current behavior. Part of SagaSmithAI/.github#6.

Boundary review

• Workflow triggers, permissions, Python/Node versions, and test commands are unchanged.
• Domain, MCP, Skills, and Workbench contracts are unchanged.
• No secrets, local state, commercial source text, or private campaign data are included.

Validation

• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.7 .github/workflows/ci.yml
• git diff --check
• Verified the selected action releases use the Node 24 runtime.

The pull-request CI run will exercise Python, Skill validation, and the UI build.

Cross-repository compatibility

The existing Core checkout and all runtime contracts remain unchanged.