agent: simplify command, schema, and repl logic

- Remove `input_schema_raw` from `SchemaInfo` and use `parameters` directly
- Simplify JSON field extraction in `Verifier` to avoid `parseArgs`
- Refactor REPL command splitting and environment substitution
- Clean up and condense comments across the codebase

src/script/Recorder.zig

@@ -129,6 +129,8 @@ fn disable(self: *Recorder, err: anyerror) void {
     }
 }
 
+// --- Tests ---
+
 fn parseLine(arena: std.mem.Allocator, line: []const u8) Command {
     return Command.parse(arena, line) catch unreachable;
 }