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.

Inizia da docs/rfcs/run-many.md e dal PR #387 collegato per comprendere la progettazione proposta del grafo di esecuzione. Risolvi le questioni aperte relative ai flag accettati, agli specificatori pkg#task e al comportamento di no-task prima di implementare il comando. Il lavoro sarà completato quando vp run-many accetterà più task e costruirà un unico grafo di esecuzione deduplicato con il comportamento documentato per dipendenze e concorrenza.