refactor(nothooks): make hook runner language-agnostic

Remove hardcoded nu interpreter. HookSpec gains an optional interpreter
field; runner infers from file extension (.sh, .nu, .py, .rb, .zsh,
.bash) when not set. Integration tests now use sh — no nu runtime dep
on CI.

crates/notcore/src/types.rs

@@ -21,6 +21,8 @@ pub struct HookSpec {
     pub name: String,
     pub script: String,
     pub phase: HookPhase,
+    #[serde(default)]
+    pub interpreter: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]