Add luacheck config + lint script

Introduce a tuned .luacheckrc and scripts/lint.sh so the engine has a
standing static-analysis baseline -- the tool that would have caught both
bugs in the previous commit before they shipped.

The config is high-signal by design: it keeps the categories that catch
real defects (undefined globals/locals, unused values, unreachable code)
and mutes the cosmetic ones the codebase deliberately lives with (a self/dt
an interface requires but a method ignores, documented empty fall-through
branches, long lines). It marks `love` mutable (games assign callbacks onto
it) and teaches it LuaJIT's table.unpack.

.luacheckrc is tracked via a .gitignore exception, matching how .github and
.gitignore opt out of the blanket dotfile ignore.

`luacheck src` now reports 7 benign warnings and 0 errors, down from 185.
Also drop one dead `require` (ItemEffects loaded src.pokemon.Pokemon and
never used it).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6bFAiQyZ5jDmewsbB4LG9

src/inventory/ItemEffects.lua

@@ -10,7 +10,6 @@
 --   "ball"                          caller must throw it (battle only)
 --   "learn", moveId                 caller must run the learn-move flow
 
-local Pokemon = require("src.pokemon.Pokemon")
 local Flags = require("src.script.Flags")
 local Strings = require("src.core.Strings")