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.

.claude/settings.json

@@ -0,0 +1,15 @@
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "bash -c 'if [ -f Cargo.toml ]; then cargo check --quiet 2>&1 | head -20; fi'"
+          }
+        ]
+      }
+    ]
+  }
+}