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.

tests/integration/tests/bootstrap.rs

@@ -186,6 +186,7 @@ fn test_setup_hooks_skipped_on_rerun() {
         name: HOOK_NAME.to_string(),
         script: script.to_str().unwrap().to_string(),
         phase: HookPhase::Setup,
+        interpreter: None,
     };
     let runner = HookRunner::new(d.to_path_buf());
     let phase_report = run_phase(&[hook_spec], &HookPhase::Setup, &runner);

tests/integration/tests/cross_crate.rs

@@ -35,6 +35,7 @@ fn test_nothooks_notsecrets_independent() {
         name: "chain".to_string(),
         script: script.to_str().unwrap().to_string(),
         phase: HookPhase::Dot,
+        interpreter: None,
     };
 
     let runner = HookRunner::new(dir.path().to_path_buf());