fix(explain): address review findings — N+1 queries, duplicate decisions, silent errors

1. fetch_open_threads: replace N+1 loop (2 queries per thread) with a
   single query using correlated subqueries for note_count and started_by.
2. extract_key_decisions: track consumed notes so the same note is not
   matched to multiple events, preventing duplicate decision entries.
3. build_timeline_excerpt_from_pipeline: log tracing::warn on seed/collect
   failures instead of silently returning empty timeline.

src/core/error.rs

@@ -28,8 +28,11 @@ pub enum ErrorCode {
     OllamaUnavailable,
     OllamaModelNotFound,
     EmbeddingFailed,
+    EmbeddingsNotBuilt,
     NotFound,
     Ambiguous,
+    HealthCheckFailed,
+    UsageError,
     SurgicalPreflightFailed,
 }
 
@@ -52,8 +55,11 @@ impl std::fmt::Display for ErrorCode {
             Self::OllamaUnavailable => "OLLAMA_UNAVAILABLE",
             Self::OllamaModelNotFound => "OLLAMA_MODEL_NOT_FOUND",
             Self::EmbeddingFailed => "EMBEDDING_FAILED",
+            Self::EmbeddingsNotBuilt => "EMBEDDINGS_NOT_BUILT",
             Self::NotFound => "NOT_FOUND",
             Self::Ambiguous => "AMBIGUOUS",
+            Self::HealthCheckFailed => "HEALTH_CHECK_FAILED",
+            Self::UsageError => "USAGE_ERROR",
             Self::SurgicalPreflightFailed => "SURGICAL_PREFLIGHT_FAILED",
         };
         write!(f, "{code}")
@@ -79,8 +85,11 @@ impl ErrorCode {
             Self::OllamaUnavailable => 14,
             Self::OllamaModelNotFound => 15,
             Self::EmbeddingFailed => 16,
+            Self::EmbeddingsNotBuilt => 21,
             Self::NotFound => 17,
             Self::Ambiguous => 18,
+            Self::HealthCheckFailed => 19,
+            Self::UsageError => 2,
             // Shares exit code 6 with GitLabNotFound — same semantic category (resource not found).
             // Robot consumers distinguish via ErrorCode string, not exit code.
             Self::SurgicalPreflightFailed => 6,
@@ -201,7 +210,7 @@ impl LoreError {
             Self::OllamaUnavailable { .. } => ErrorCode::OllamaUnavailable,
             Self::OllamaModelNotFound { .. } => ErrorCode::OllamaModelNotFound,
             Self::EmbeddingFailed { .. } => ErrorCode::EmbeddingFailed,
-            Self::EmbeddingsNotBuilt => ErrorCode::EmbeddingFailed,
+            Self::EmbeddingsNotBuilt => ErrorCode::EmbeddingsNotBuilt,
             Self::SurgicalPreflightFailed { .. } => ErrorCode::SurgicalPreflightFailed,
         }
     }