feat: include CLI version in User-Agent header

[Copilot]

The User-Agent header sent by miactl omitted the CLI version, making it impossible to track version adoption. After this change:

# versioned build
User-Agent: miactl@0.13.0 (darwin/arm64)

# unversioned/dev build (unchanged behavior)
User-Agent: miactl (darwin/arm64)

Changes

• internal/version/version.go — new shared package holding Version and BuildDate vars, set via ldflags at build time
• internal/cmd/version.go — removed local Version/BuildDate declarations; now imports from internal/version
• internal/clioptions/clioptions.go — defaultUserAgent() appends @<version> to the command name when version.Version is non-empty
• Makefile — VERSION_MODULE_NAME updated to github.com/mia-platform/miactl/internal/version; goreleaser picks this up from the environment automatically