feat(pets): pet engine + display.pet config

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.

hermes_cli/config.py

@@ -1633,6 +1633,31 @@ DEFAULT_CONFIG = {
             "fields": ["model", "context_pct", "cwd"],  # Order shown; drop any to hide
         },
         "copy_shortcut": "auto",  # "auto" (platform default) | "ctrl_c" | "ctrl_shift_c" | "disabled"
+        # Petdex animated mascot (https://github.com/crafter-station/petdex).
+        # A purely cosmetic sprite that reacts to agent activity across the
+        # CLI, TUI, and desktop app. Manage with `hermes pets`. Disabled until
+        # a pet is installed + selected (no effect on prompt caching — this is
+        # a display concern only).
+        "pet": {
+            "enabled": False,
+            # Active pet slug; resolved against installed pets in
+            # get_hermes_home()/pets/. Empty → first installed pet.
+            "slug": "",
+            # Terminal render protocol for CLI/TUI:
+            #   auto  — detect kitty/iTerm2/sixel, else unicode half-blocks
+            #   kitty | iterm | sixel | unicode | off
+            "render_mode": "auto",
+            # Master size scalar (relative to native 192×208 frames). One knob
+            # shrinks every surface: the desktop canvas scales its pixels by it
+            # and the CLI/TUI derive their terminal column width from it. The
+            # half-block fallback clamps to a legibility floor (it can't shrink
+            # as far as true-pixel kitty/GUI without turning to mush).
+            "scale": 0.33,
+            # Hard override for terminal column width. 0 = auto (derive from
+            # scale); set a positive int only to pin the half-block/kitty width
+            # independently of scale.
+            "unicode_cols": 0,
+        },
     },
 
     # Web dashboard settings