feat: add parallel reusable workflow for faster CI

[kim-em]

This PR adds a new reusable workflow that enables parallel execution of CI checks on separate runners:

• Build job runs first: elan setup, config, mathlib cache, lake build
• Parallel jobs run after build: test, lint, lean4checker, reservoir
• Each parallel job restores the .lake cache from the build job

                    ┌─────────────────┐
                    │     build       │
                    └────────┬────────┘
                             │
         ┌───────────┬───────┴───────┬───────────┐
         ▼           ▼               ▼           ▼
    ┌─────────┐ ┌─────────┐  ┌────────────┐ ┌───────────┐
    │  test   │ │  lint   │  │lean4checker│ │ reservoir │
    └─────────┘ └─────────┘  └────────────┘ └───────────┘

Usage

jobs:
  ci:
    uses: leanprover/lean-action/.github/workflows/ci.yml@v1
    with:
      test: "true"
      lint: "true"
      lean4checker: "true"

Backward Compatibility

The existing action.yml composite action is unchanged. Users can choose between:

• Sequential (existing): uses: leanprover/lean-action@v1
• Parallel (new): uses: leanprover/lean-action/.github/workflows/ci.yml@v1

🤖 Prepared with Claude Code