fix: ValidationError.path optional, correct position_quantity usage in prompts

- ValidationError.path is Option<String> — the API omits it for top-level
  structural errors. The required String was causing every validate call to
  fail to deserialize, falling through to submission instead of catching errors.

- Log path as "(top-level)" when absent

- Prompts: add explicit CRITICAL note that {"method":"position_quantity"} is
  wrong — position_quantity is an Expr (uses "kind") not a SizingMethod (uses
  "method"). The new SizingMethod examples caused the model to over-apply
  "method" to exits universally across the entire run.

- Prompts: note that fixed_sum has no multiplier field (additionalProperties)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/agent.rs

@@ -345,11 +345,11 @@ pub async fn run(cli: &Cli) -> Result<()> {
         match swym.validate_strategy(&strategy).await {
             Ok(api_errors) if !api_errors.is_empty() => {
                 for e in &api_errors {
-                    warn!("  DSL error at {}: {}", e.path, e.message);
+                    warn!("  DSL error at {}: {}", e.path.as_deref().unwrap_or("(top-level)"), e.message);
                 }
                 let error_notes: Vec<String> = api_errors
                     .iter()
-                    .map(|e| format!("DSL error at {}: {}", e.path, e.message))
+                    .map(|e| format!("DSL error at {}: {}", e.path.as_deref().unwrap_or("(top-level)"), e.message))
                     .collect();
                 validation_notes.extend(error_notes);
                 let record = IterationRecord {

src/prompts.rs

@@ -110,8 +110,13 @@ Buy a fixed number of base units (semantic alias for a decimal string):
 Alternatively, `"9999"` works for exits: sell quantities are automatically capped to the open
 position size, so a large fixed number is equivalent to `position_quantity`.
 
-NEVER use placeholder strings like `"ATR_SIZED"`, `"FULL_BALANCE"`, `"all"`, `"dynamic"` —
-these are rejected immediately.
+CRITICAL mistakes to never make:
+- `{{"method":"position_quantity"}}` is WRONG — `position_quantity` is an Expr, not a SizingMethod.
+  CORRECT: `{{"kind":"position_quantity"}}`. The `"method"` field belongs ONLY to the three
+  declarative sizing objects (`fixed_sum`, `percent_of_balance`, `fixed_units`).
+- `{{"method":"fixed_sum","amount":"100","multiplier":"2.0"}}` is WRONG — `fixed_sum` has no
+  `multiplier` field. Only `amount` is accepted alongside `method`.
+- NEVER add extra fields to SizingMethod objects — they use `additionalProperties: false`.
 
 ### Multi-timeframe
 Any expression can reference a different timeframe via "timeframe" field.

src/swym.rs

@@ -14,7 +14,8 @@ pub struct ValidationResponse {
 
 #[derive(Debug, Deserialize, Clone)]
 pub struct ValidationError {
-    pub path: String,
+    /// Dotted JSON path to the offending field. Absent for top-level structural errors.
+    pub path: Option<String>,
     pub message: String,
 }