feat(platform): extract non-chat apps into potato-os/apps

[slomin]

Summary

• Extract Permitato and Skeleton from potato-os/core into the new potato-os/apps repo
• Add POTATO_APPS_REPO env var to install_dev.sh and prepare_imager_bundle.sh for sourcing apps from the external repo
• Chat remains the only built-in app in core
• Zero platform code changes — app discovery, supervisor, lifecycle, routes, and shell all work without modification

Changes

Apps repo (potato-os/apps):

• Permitato (20+ source files, 13 Python + 6 Playwright tests) and Skeleton copied in
• pytest.ini, playwright.config.js, requirements-dev.txt added
• AGENTS.md with app contract docs and deployment instructions
• GitHub Actions CI for Python tests

Core repo (this PR):

• apps/permitato/ and apps/skeleton/ deleted
• bin/install_dev.sh — added POTATO_APPS_REPO support, removed skeleton keep-always
• bin/prepare_imager_bundle.sh — added POTATO_APPS_REPO support, removed skeleton keep-always
• bin/publish_ota_release.sh — removed --exclude='apps/permitato' (nothing to exclude)
• playwright.config.js — added testIgnore for .cache, .claude, references
• tests/ui/icon-rail.spec.js — mock multi-app response for switcher tests
• Removed 14 tests referencing extracted files, added 3 tests for POTATO_APPS_REPO
• AGENTS.md updated to reference potato-os/apps

Test evidence

577 passed in 10.19s     (pytest)
118 passed (32.5s)       (playwright)
274 passed in 0.86s      (apps repo pytest)

Deployment

# Chat only (default)
./bin/install_dev.sh

# Chat + Permitato from external repo
POTATO_APPS_REPO=/path/to/apps-repo \
  POTATO_IMAGE_APPS=chat,permitato \
  ./bin/install_dev.sh

Closes #282