Pin grammar rev to scaffold commit

Zed's [grammars.*] block requires a real Git SHA — "HEAD" does not
resolve. Lock to 3f209ef so a fresh `zed: install dev extension` can
clone the grammar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

extension.toml

@@ -12,4 +12,4 @@ repository = "https://github.com/kistaro/zed-p8"
 # this should point at a public Git URL.
 [grammars.p8_cart]
 repository = "file:///Users/norberg/gitea-repos/zed-p8"
-rev = "HEAD"
+rev = "3f209efa897558e8ecd7aa3612846dc12798b0bb"