Let the screensaver pin one ttfx effect via shell.json

[buddha9]

Summary

• The screensaver has always picked a random ttfx effect (beams, matrix, fireworks, ...) each cycle, with no way to stick to one you like.
• Adds an optional idle.screensaverEffect key to shell.json. When set, it constrains ttfx's --random-effect selection to that one effect via --include-effects; an unrecognized name falls back to random and sends a notification saying why.
• New hidden helper bin/omarchy-screensaver-effect resolves and validates the configured effect, kept separate from bin/omarchy-screensaver so the logic is unit-testable without touching the live screensaver loop (mirrors how IdleModel.js is split out from Service.qml).
• Docs updated in manual/13-toggles-idle-screensaver.md, docs/omarchy-shell.md, and the bundled omarchy agent skill's plugins.md.

Test plan

• test/shell.d/screensaver-effect-test.sh (new): unset falls back to random silently, a valid effect resolves and is passed through, an invalid effect falls back to random and notifies.
• ./test/all run locally — no new failures (5 pre-existing failures in this sandbox are all due to a missing omarchy-pkgs checkout or environment-specific rendering/locale issues, unrelated to this change).
• Manual check on a real Omarchy install: set "screensaverEffect": "matrix" in ~/.config/omarchy/shell.json, trigger the screensaver (Super + Esc), confirm it always renders the matrix digital-rain effect instead of a random pick.

🤖 Generated with Claude Code