MCP: complete tool hints and check nested argument keys

Extend the existing MCP tools instead of adding a second set (#259):

- every tool states destructiveHint and idempotentHint next to
  readOnlyHint/openWorldHint; input schemas are closed
- the #776 unknown-key check walks the schema, so a misspelt key in a
  batch step, a pointer event or mods is -32602 with its path instead
  of being ignored
- script::parse refuses JSON step keys other than command/params
- a panic outside a command (rendering, inspection, the app connection)
  is a tool error and the server keeps running

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/engine/src/script.rs

@@ -11,7 +11,7 @@
 //! **Script text** ([`parse`]): a JSON array of steps, JSON lines (`{"command": …, "params": …}`),
 //! or compact lines `command.id {json}` / `command.id={json}` / `command.id`; blank lines and
 //! lines starting with `#` or `//` are ignored. A leading byte order mark and CRLF line endings are
-//! accepted.
+//! accepted. A JSON step takes only `command` and `params`; any other key is an error.
 
 use serde_json::{Value, json};
 
@@ -59,6 +59,10 @@ pub fn parse(text: &str) -> Result<Vec<Step>, String> {
 }
 
 fn step_of(v: &Value) -> Result<Step, String> {
+    // A misspelt key (`parms`) would otherwise run the command with its defaults.
+    if let Some(key) = v.as_object().and_then(|o| o.keys().find(|k| !matches!(k.as_str(), "command" | "params"))) {
+        return Err(format!("unknown key `{key}` (a step takes `command` and `params`)"));
+    }
     let command = v.get("command").and_then(Value::as_str).filter(|s| !s.is_empty()).ok_or("missing `command`")?.to_string();
     let params = v.get("params").cloned().filter(|p| !p.is_null()).unwrap_or(json!({}));
     if !params.is_object() {
@@ -195,6 +199,16 @@ mod tests {
         assert!(parse("frame.create {bad").unwrap_err().contains("line 1"));
     }
 
+    /// A misspelt step key ran the command with its defaults.
+    #[test]
+    fn a_step_with_an_unknown_key_is_refused() {
+        for text in [r#"[{"command": "file.new", "parms": {"width": 300}}]"#, r#"{"command": "file.new", "parms": {"width": 300}}"#] {
+            let e = parse(text).unwrap_err();
+            assert!(e.contains("unknown key `parms`"), "{e}");
+        }
+        assert!(parse(r#"[{"command": "file.new", "params": {"width": 300}}]"#).is_ok());
+    }
+
     /// Windows editors and PowerShell 5.1 save "UTF-8 with BOM" with CRLF line endings; the BOM
     /// became part of the first command id (#654).
     #[test]