Fix synthesis truncation and trace masking (#16, #19)

The synthesis step was passing max_tokens=4096 to Claude, which was
not enough for a full ResearchResult JSON over a real evidence set
(28 sources). The model's output got cut mid-string, json.loads
failed, and the agent fell back to a stub answer with zero citations.

The trace logger then truncated the raw_response to 1000 chars before
recording it, hiding the actual reason for the parse failure (the
truncated JSON suffix) and making the bug invisible from traces.

Fixes:
- Bump synthesis max_tokens to 16384
- Capture and log Claude's stop_reason on synthesis_error so future
  truncation cases are diagnosable from the trace alone
- Log the parser exception text alongside the raw_response
- Stop slicing raw_response — record the full string

Verified end-to-end against the Utah crops question:
- Before: 0 citations, confidence 0.10, fallback stub
- After:  9 citations, confidence 0.88, real synthesized answer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

researchers/web/agent.py

@@ -448,7 +448,7 @@ class WebResearcher:
 
         response = self.client.messages.create(
             model=self.model_id,
-            max_tokens=4096,
+            max_tokens=16384,
             messages=[{"role": "user", "content": prompt}],
         )
 
@@ -457,6 +457,7 @@ class WebResearcher:
 
         # Parse the JSON response
         raw_text = response.content[0].text.strip()
+        stop_reason = response.stop_reason
         # Strip markdown fences if the model added them despite instructions
         if raw_text.startswith("```"):
             raw_text = raw_text.split("\n", 1)[1] if "\n" in raw_text else raw_text[3:]
@@ -465,11 +466,16 @@ class WebResearcher:
 
         try:
             data = json.loads(raw_text)
-        except json.JSONDecodeError:
+        except json.JSONDecodeError as parse_err:
             trace.log_step(
                 "synthesis_error",
-                decision="Failed to parse synthesis JSON, returning fallback",
-                raw_response=raw_text[:1000],
+                decision=(
+                    f"Failed to parse synthesis JSON ({parse_err}); "
+                    f"stop_reason={stop_reason}"
+                ),
+                stop_reason=stop_reason,
+                parse_error=str(parse_err),
+                raw_response=raw_text,
             )
             return self._fallback_result(
                 question, evidence, trace, total_tokens, iterations,