session-memory: recurring-error signal + clustering (WP-0006 T02)

detect/signals.py sig_recurring_error emits one signal per distinct error
fingerprint per session (magnitude = in-session occurrences), so the same error
recurring across sessions/repos/flavors clusters into a candidate root-cause
problem pattern via the existing clusterer — cross-flavor flagged automatically.
3 new tests; suite 98/98 green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

session_memory/detect/signals.py

@@ -156,10 +156,28 @@ def sig_tool_thrash(digest, ctx) -> list[Signal]:
     return []
 
 
+def sig_recurring_error(digest, ctx) -> list[Signal]:
+    """Problem: a normalized error fingerprint (WP-0006) — one signal per distinct
+    error in the session, so the same error across sessions/repos/flavors clusters
+    into a candidate root-cause pattern (locus = fingerprint, magnitude = in-session
+    occurrences). This is the content-level 'why', not just a coarse error count.
+    """
+    out: list[Signal] = []
+    for snip in digest.get("error_snippets", []) or []:
+        fp = snip.get("fingerprint")
+        if not fp:
+            continue
+        out.append(_base(digest, "recurring_error", PROBLEM, fp, float(snip.get("count", 1)),
+                         sample=snip.get("sample", ""), tool=snip.get("tool"),
+                         occurrences=snip.get("count", 1)))
+    return out
+
+
 EXTRACTORS: list[Callable] = [
     sig_retry_storm, sig_repeated_errors, sig_budget_overrun, sig_abandoned,
     sig_clean_pass, sig_error_then_recovery,
     sig_infra_overhead, sig_schema_thrash, sig_tool_thrash,
+    sig_recurring_error,
 ]