Allow CEO agents to update company branding (name, description, logo, color)

- Add updateCompanyBrandingSchema restricting agent-updatable fields to name,
  description, brandColor, and logoAssetId
- Update PATCH /api/companies/:companyId to allow CEO agents with branding-only
  fields while keeping admin fields (status, budget, etc.) board-only
- Allow agents to GET /api/companies/:companyId for reading company info
- issuePrefix (company slug) remains protected — not in any update schema
- Document branding APIs in SKILL.md quick reference and api-reference.md

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared/src/validators/index.ts

@@ -15,8 +15,10 @@ export {
 export {
   createCompanySchema,
   updateCompanySchema,
+  updateCompanyBrandingSchema,
   type CreateCompany,
   type UpdateCompany,
+  type UpdateCompanyBranding,
 } from "./company.js";
 export {
   portabilityIncludeSchema,