diag(1101): log classify verdict + matched trigger on merge gate failures

Bug 1101's reframed AC1: when a non-success merge runs, log the typed
GateFailureKind, the matched classifier-trigger substring (if any) and
~90 chars of surrounding context. Fires on every gate failure regardless
of routing, so the next fixup-loop bounce will tell us which substring is
fooling classify() into Fmt|Lint|SourceMapCheck on what's actually a Test
failure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/agents/pool/pipeline/merge/runner.rs

@@ -186,6 +186,50 @@ impl AgentPool {
                         .map(|k| k.is_self_evident_fix())
                         .unwrap_or(false);
 
+                // Bug 1101 diagnostic: log the classified failure_kind and the
+                // matched classifier-trigger substring with surrounding context,
+                // so we can confirm whether classify() is incorrectly matching
+                // a passing-step stdout substring (e.g. "Diff in " inside a
+                // failing test's panic message) and bouncing the story to a
+                // fixup coder. Remove once the fix lands.
+                if let Ok(r) = report.as_ref()
+                    && let crate::agents::merge::MergeResult::GateFailure {
+                        output: gate_output,
+                        failure_kind: Some(k),
+                    } = &r.result
+                {
+                    const TRIGGERS: &[&str] = &[
+                        "CONFLICT (content):",
+                        "Merge conflict:",
+                        "Diff in ",
+                        "would reformat",
+                        "missing-docs direction",
+                        "error[clippy::",
+                        "warning[clippy::",
+                        "missing_doc_comments",
+                        "error[E",
+                    ];
+                    let matched = TRIGGERS
+                        .iter()
+                        .find_map(|t| gate_output.find(t).map(|i| (*t, i)));
+                    let (trigger, context) = match matched {
+                        Some((t, i)) => {
+                            let start = i.saturating_sub(30);
+                            let end = (i + t.len() + 60).min(gate_output.len());
+                            let ctx = gate_output
+                                .get(start..end)
+                                .unwrap_or("<context unavailable>")
+                                .replace('\n', " ");
+                            (Some(t), ctx)
+                        }
+                        None => (None, String::from("<no trigger matched>")),
+                    };
+                    slog!(
+                        "[merge] classify diagnostic for '{sid}': failure_kind={k:?} \
+                         is_fixup={is_fixup} trigger={trigger:?} context='{context}'"
+                    );
+                }
+
                 if is_no_commits {
                     let reason = kind.display_reason();
                     if let Err(e) = crate::agents::lifecycle::transition_to_blocked(&sid, &reason) {