Implement `run-many`

Tracking issue for the run-many RFC.

RFC: docs/rfcs/run-many.md (PR #387)

Summary

Add vp run-many <task1> <task2> ... that builds one execution graph containing multiple requested tasks, deduped by node. This is the schedule that today's primitives can't express:

• ["vp run a", "vp run b"] (#381) serializes the two (each vp run is its own isolated graph, items run one at a time).
• vp run --parallel only takes one task and drops every dependency edge — too blunt when some edges matter.

Combined with the string[] shorthand from #381, the tree of execution graphs gains a full algebra: string[] sequences siblings, run-many fans out within one node.

Open questions

• Which run flags are accepted by run-many (the RFC marks this TBD).
• pkg#task specifiers — included or restricted to bare task names for v1.
• Behavior when invoked with no task names (error vs. interactive multi-select).

See the RFC for the full motivation, diagrams, and a worked composition example.

Start with docs/rfcs/run-many.md and the linked PR #387 to understand the proposed execution-graph design. Resolve the open questions about accepted flags, pkg#task specifiers, and no-task behavior before implementing the command. Done means vp run-many accepts multiple tasks and builds one deduplicated execution graph with the documented dependency and concurrency behavior.