feat(skillhub): add default system skills

[GatewayJ]

Summary

• add an embedded system skill source for default skills
• expose built-in skills in Skill Hub and prevent deleting or uploading over them
• install default system skills during agent create, start, and recreate flows
• move skill-installer out of runtime templates into the shared system skill source

Validation

• go test ./internal/api ./internal/skillhub ./internal/agent ./internal/templates
• go test ./...
• pnpm --dir web/app check
• git diff --check
• make build

[GatewayJ]