Add full and sandboxed Agent Grid skills

[emilio-balda]

Summary:

• add agent-grid-full for agents with a shell, network access, and Git
• add agent-grid-sandboxed for agents that use MCP file tools without Git or zip uploads
• document only the artifact-create files flow in both new skills
• add public mirrors for both new skills while keeping the original Anima mirror and surface unchanged
• use the Agent Grid logo for both new skills
• give all three skills helpful default prompts

Validation:

• Not run, as requested. The final diff received a manual scope, schema, and ASD-STE100 review.

[emilio-balda]

Hey @macabeus, not sure what would be the consequences of merging something like this.
I currently use this branch only to generate an agent-grid-sandboxed.zip file for ChatGPT's plugin submission.
If it doesn't hurt we could merge these changes, but not if it can cause problems.