feat: support stack names in bulk commands

[omattsson]

Context

With #40 (name-based stack resolution), single-stack commands now accept <name|id>. However, bulk deploy, bulk stop, bulk clean, and bulk delete still use parseBulkIDs which only accepts positive integers.

Problem

parseBulkIDs in cli/cmd/root.go splits on commas and calls strconv.Atoi, rejecting both UUIDs and stack names. This creates an inconsistency where stackctl stack deploy my-stack works but stackctl stack bulk deploy my-stack,other-stack does not.

Proposed solution

Replace parseBulkIDs with a bulk resolver that:

1. Splits on commas
2. Runs each element through resolveStackID
3. Deduplicates resolved IDs
4. Returns a slice of resolved IDs

This would align bulk commands with single-stack commands.

Follow-up from

#40 code review item #7