fix: log R1 thinking, catch repeated DSL errors, add unsupported indicators

Three improvements from the 2026-03-09T18:45:04 run analysis:

**R1 thinking visibility (claude.rs, agent.rs)**
extract_think_content() returns the raw <think> block content before it
is stripped. agent.rs logs it at DEBUG level so 'RUST_LOG=debug' lets
you see why the model keeps repeating a mistake — currently the think
block is silently discarded after stripping.

**Prompt: unsupported indicators and bollinger_upper Expr mistake (prompts.rs)**
- bollinger_upper / bollinger_lower used as {"kind":"bollinger_upper",...}
  was the dominant failure in iters 9-15. Added explicit correction:
  use {"kind":"func","name":"bollinger_upper","period":20} in Expr context,
  never as a standalone kind.
- roc, hma, vwap, macd, cci, stoch are NOT in the swym schema. Added a
  clear "NOT supported" list alongside the supported func names.

**Repeated API error detection in diagnose_history (agent.rs)**
If the same "unknown variant `X`" error appears 2+ times in the last 4
iterations, a targeted diagnosis note is emitted naming the bad variant
and pointing to the DSL reference. This surfaces in the next iteration
prompt so the model gets actionable feedback before it wastes another
backtest budget on the same mistake.

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

src/prompts.rs

@@ -151,6 +151,12 @@ Common mistakes to NEVER make:
   `bollinger_upper`, `bollinger_lower`.
 - `volume` is a candle FIELD, not a func name. Access it as `{{"kind":"field","field":"volume"}}`.
   To compute EMA of volume: `{{"kind":"apply_func","name":"ema","period":20,"expr":{{"kind":"field","field":"volume"}}}}`.
+- `bollinger_upper` and `bollinger_lower` are FUNC NAMES, not Expr kinds. To compare close to the upper band:
+  `{{"kind":"compare","left":{{"kind":"field","field":"close"}},"op":">","right":{{"kind":"func","name":"bollinger_upper","period":20}}}}`
+  NEVER write `{{"kind":"bollinger_upper",...}}` — `bollinger_upper` is not an Expr kind.
+- `roc` (rate of change), `hma` (Hull MA), `vwap`, `macd`, `cci`, `stoch` are NOT supported.
+  Use `sma`, `ema`, `wma`, `rsi`, `atr`, `adx`, `supertrend`, `std_dev`, `sum`, `highest`, `lowest`,
+  `bollinger_upper`, `bollinger_lower` only.
 
 ## Working examples