fix(prompts): forbid dynamic quantity expressions, require plain decimal string

The model was generating Expr objects for quantity (e.g. ATR-based sizing),
causing consistent QuantitySpec deserialization failures. Replace the
"prefer dynamic sizing" hint with an explicit rule: quantity must always
be a fixed decimal string like "0.001".

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

src/prompts.rs

@@ -50,9 +50,9 @@ unrealised_pnl — current unrealised P&L
 bars_since_entry — complete bars elapsed since position was opened
 balance — free balance of a named asset (e.g. "usdt", "usdc")
 
-### Dynamic quantity
+### Quantity
-Action quantity can be a fixed string ("0.001") or an Expr for dynamic sizing.
+Action quantity MUST be a fixed decimal string, e.g. `"quantity": "0.001"`.
-ATR-based sizing, percent-of-balance, etc.
+NEVER use an expression object for quantity — only plain decimal strings are accepted.
 
 ### Multi-timeframe
 Any expression can reference a different timeframe via "timeframe" field.
@@ -311,6 +311,7 @@ Common mistakes to NEVER make:
 - Don't ignore fees — a strategy needs to overcome 0.1% per round trip
 - Always gate buy rules with position state "flat" and sell rules with "long"
 - Never add a short-entry (sell when flat) rule — spot markets are long-only
+- Never use an expression object for `quantity` — it must always be a plain decimal string like `"0.001"`
 "##
     )
 }