ENG-5435: Add region list and get commands to CLI

[lukaszo]

Summary

Adds two new subcommands to the CLI:

• koyeb regions list - Lists all available regions with their ID, name, scope, and volumes enabled status. Only regions with "available" status are shown.
• koyeb regions get <ID> - Shows detailed information about a specific region including instances available in that region.

Example output

koyeb regions list

ID      NAME            SCOPE           VOLUMES ENABLED
ap      Asia Pacific    continental     false
dal     Dallas          metropolitan    true

koyeb regions get was

ID:              was
Name:            Washington, D.C
Scope:           metropolitan
Volumes enabled: true

Instances:
  - 2xlarge
  - 3xlarge
  - 4xlarge
  ...

Both commands support JSON and YAML output formats via -o json / -o yaml.

Changes

• pkg/koyeb/regions.go - Region command and handler setup
• pkg/koyeb/regions_list.go - List command using CatalogRegionsApi.ListRegions, filtering to available regions only
• pkg/koyeb/regions_get.go - Get command using CatalogRegionsApi.GetRegion with custom plain-text detail view for table output
• pkg/koyeb/koyeb.go - Register NewRegionCmd() in root command
• Makefile - Include regions (and other missing commands) in doc generation
• docs/reference.md - Regenerated docs

Testing

• go build ./... ✅
• go test ./... ✅ (all tests pass)
• gofmt ✅ (no formatting issues)
• Note: make lint has a pre-existing toolchain mismatch issue on master (go1.26 vs golangci-lint built with go1.25)

Linear issue: ENG-5435