install_pet now refuses spritesheet/pet.json URLs that aren't on a petdex
host (matching thumbnail_png's existing _is_petdex_host guard), so a
spoofed manifest can't redirect a download at an arbitrary host. Slugs
are normalized to a single path segment before indexing into pets_dir(),
closing a path-traversal vector in load_pet/remove_pet/install_pet.
Add the shared pet engine under agent/pet/: spritesheet manifest loading
and in-process caching, six-state animation model, frame rendering, and
the persistent pet store. Register the display.pet config block (pet,
scale, enabled, etc.) that every surface reads from. Covered by
tests/agent/test_pet_engine.py.
