Prepare Ghostable v3 CLI release

[joerucci]

What changed

• move the Go module and internal imports to github.com/ghostable-dev/ghostable/v3
• publish future npm releases to the existing @ghostable/cli package
• route prereleases to npm next and stable releases to npm latest
• mark GitHub release candidates as prereleases and keep them from becoming latest
• skip Homebrew cask uploads automatically for prerelease tags
• standardize GitHub release titles as vX.Y.Z Release
• update install documentation and security wording for the official CLI transition

Release behavior

Tag	GitHub	npm	Homebrew
v3.0.0-rc.1	prerelease	@ghostable/cli@next	unchanged
v3.0.0	latest	@ghostable/cli@latest	updated

This PR does not create a tag or publish a release. The existing @ghostable/beta package remains untouched until stable v3 has been published and verified.

Validation

• make check
• make security-check
• YAML parsing for CI and release workflows
• git diff --check
• version-injected v3 binary smoke test
• all six GoReleaser target cross-builds
• release secret name verification
• v3 release-channel and existing-tag guard checks

The authenticated GoReleaser Pro configuration check will also run on main after merge, as configured by the existing CI workflow.