agent-platform release v4.104.0-rc.1

Added

• (kagent) Register kagent’s MCP server with muster in #760 by @teemow

Changed

• Release branches publish through the tag pipeline only in #761 by @teemow
• Release branches skip the branch-only jobs like main in #763 by @teemow

Full Changelog: https://github.com/giantswarm/agent-platform/compare/v4.103.2...v4.104.0-rc.1