Release v0.9.0 Planning

[kmannislands]

This is a ticket to track:

• Deprecate EARTHLY_* name
• Buildkit upstream update
• OTEL
• Better error messages
• AI compatibility? Agents.md/Claude code skill/MCP
• Publish a renovate config for projects using earthbuild
• IDE tooling

Other ideas to discuss:

• RETURN statement syntax
• INCLUDE statement similar to IMPORT but inline