docs: add RFC for run-many command

[branchseer]

Summary

Add RFC documentation for the run-many command, a new feature that enables running multiple tasks as a single execution graph with dependency-aware parallelism.

Changes

• Added docs/rfcs/run-many.md with comprehensive RFC covering:
  • Explanation of vite-task's two scheduling structures: ExecutionGraph (DAG with concurrency limit) and tree (recursive graphs)
  • Problem statement: current inability to run multiple tasks as one DAG while respecting inter-task dependencies
  • run-many solution: builds a single graph with multiple requested roots, deduplicating nodes
  • Composition patterns showing how run-many combines with command: [...] sequencing for flexible CI workflows
  • Visual diagrams (Mermaid flowcharts) illustrating scheduling behavior

Details

The RFC documents how vp run-many build test differs from alternatives:

• Unlike --parallel, it preserves real dependency edges (e.g., test → build)
• Unlike ["vp run build", "vp run test"], it allows tasks to start as soon as their dependencies finish, not waiting for all builds to complete
• Shows practical composition in CI tasks like ["vp run prepare", "vp run-many lint test build typecheck", "vp run publish"]

This is a design document only; no implementation changes are included.

https://claude.ai/code/session_017Mzp1DyTE1ZDAEWZCi4QZF