script: strip backendNodeId and fix error line reporting

- Allow recording commands with backendNodeId if they also have a
  selector, stripping backendNodeId during formatting.
- Point UnterminatedQuote errors to the opening line instead of EOF.
- Fix buildHints allocation when fields is empty but required is not.
- Update docs to clarify positional argument rules.

docs/agent.md

@@ -81,13 +81,16 @@ syntax: anything that doesn't match those three forms is a parse error.
 Slash commands accept any of:
 
 - a single positional value, when the tool has exactly one required field —
-  `/goto 'https://example.com'`, `/click selector='Login'`,
-  `/extract '{"karma":"#karma"}'`;
+  `/goto 'https://example.com'`, `/extract '{"karma":"#karma"}'`;
 - `key=value` pairs — values may be bare or quoted; strings with whitespace
   must be quoted (`/fill selector='#email' value='user@x.com'`);
 - a raw `{json}` blob — handed straight to the tool (`/findElement
   {"role":"button"}`).
 
+Tools whose selector is optional (e.g. `/click`, `/hover`, `/findElement`)
+have zero required fields, so they don't take a positional and must be
+written as `key=value`: `/click selector='Login'`, not `/click 'Login'`.
+
 Quoting is content-aware: `'…'`, `"…"`, and triple-quoted `'''…'''` /
 `"""…"""` for values that mix both quote styles or span multiple lines.
 Recorded scripts round-trip through the parser without escapes.