fix(observer): narrow parallel_session detector to tool_result evidence (C-2)

extractEnvironment was scanning JSON.stringify(turn) for collision markers
(чужой staged / foreign git index / index.lock / another git process). Prose
mentions in user/assistant text flipped parallel_session=true. Live FP proven
on episodes-2026-05.jsonl line 20: my own analysis turn was non-parallel but
recorded parallel_session: true because the finding text mentioned the markers.

Fix: collectToolResultText(turn) — gather text only from tool_result blocks
(both string content and structured `[{type:text,text}]` arrays). Scan THAT
for collision markers; prose is no longer a signal.

Tests: rewrote `parallel_session narrowed` block — false on user/assistant
prose / no-tool-result turns; true on tool_result strings + structured form.
106/106 parser tests green.

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

tools/observer-transcript-parser.mjs

@@ -165,11 +165,41 @@ export function extractEnvironment(allEntries, turnStartIdx) {
 
   // Only strong collision evidence — a bare mention of "parallel sessions" is
   // not a signal (best-effort per spec R2; prefer false-negative over false-positive).
-  const parallel_session = /чужой staged|foreign git index|index\.lock|another git process/i.test(rawTurn);
+  // Scope NARROWED to tool_result content (real command output / Bash stderr): prose
+  // mentions in user prompts / assistant text — including analysis text that
+  // references collision phrases — must not trigger. Fixes live FP (episode line 20).
+  const parallel_session = /чужой staged|foreign git index|index\.lock|another git process/i.test(
+    collectToolResultText(turn)
+  );
 
   return { economy_level, model, post_compaction, session_turn, parallel_session };
 }
 
+/**
+ * Collect text content from tool_result blocks in the turn — the only surface
+ * trusted for parallel_session collision evidence (see extractEnvironment).
+ * Supports both string content and the structured array form
+ * (`content: [{ type: 'text', text }]`).
+ */
+function collectToolResultText(turn) {
+  const parts = [];
+  for (const e of turn) {
+    const content = e && e.message && Array.isArray(e.message.content) ? e.message.content : [];
+    for (const b of content) {
+      if (!b || b.type !== 'tool_result') continue;
+      const c = b.content;
+      if (typeof c === 'string') {
+        parts.push(c);
+      } else if (Array.isArray(c)) {
+        for (const sub of c) {
+          if (sub && typeof sub.text === 'string') parts.push(sub.text);
+        }
+      }
+    }
+  }
+  return parts.join('\n');
+}
+
 /** Task size: total tool calls + unique file paths touched (per spec §3, gap-resolution 2). */
 export function extractTaskSize(turn) {
   let tool_calls = 0;