Ignore a .lp-cache symlink, not just a directory

A fresh worktree has no V8 cache, and the way to avoid a 10-minute
rebuild is to symlink `.lp-cache` at the main checkout's. `/.lp-cache/`
matches only a directory, so that symlink shows up as untracked and a
`git add -A` stages it -- an absolute path that dangles anywhere else,
and one `build.zig` picks up when present.

.gitignore

@@ -1,5 +1,5 @@
 /.zig-cache/
-/.lp-cache/
+/.lp-cache
 /.lp-history
 /.lp-history.js
 /.lp-agent.zon