feat(system): DPLAN-0133 Phase 1 (part 1/2): add gitignore rule for apps/integrations/** with README negation, plus scaffold README.md in all 10 core branches' apps/integrations/ — private integration space is now leak-proof by construction. Drivers and wrappers dropped here stay local; only README.md is tracked. See DPLAN-0133 for architecture rationale (three-layer design: @api drivers, per-branch wrappers, public generic contracts). #289