feat: surface base + persona system prompts in observer feed (#1103)

Signed-off-by: Will Pfleger <pfleger.will@gmail.com>
Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Co-authored-by: npub1mn7jgtj4w2pd0g0zeuhxsa6jy6p0rewxz4kujt98my82ahfmp72sxjexk7 <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@sprout-oss.stage.blox.sqprod.co>

desktop/src/features/agents/ui/agentSessionTranscriptHelpers.ts

@@ -49,6 +49,46 @@ export function parsePromptText(text: string): {
   };
 }
 
+/**
+ * Split the framed `session/new` `systemPrompt` into its `Base`/`System`
+ * sub-sections deterministically.
+ *
+ * The harness frames the value as `[Base]\n{base}\n\n[System]\n{persona}`, with
+ * either prompt omitted when absent: base-only is `[Base]\n{base}`, persona-only
+ * is `[System]\n{persona}`. We partition on the FIRST `\n[System]\n` boundary and
+ * read each labeled body literally. Unlike the generic `parsePromptSections`,
+ * embedded `[...]` lines inside a body never start a new section — so a persona
+ * containing a bracketed line, or a mid-string-elided header on an oversize
+ * prompt, can never drop a label or inflate the section count.
+ */
+export function parseSystemPromptSections(
+  systemPrompt: string,
+): PromptSection[] {
+  const sections: PromptSection[] = [];
+
+  // Persona-only frame: no [Base], starts directly with [System].
+  if (systemPrompt.startsWith("[System]\n")) {
+    const body = systemPrompt.slice("[System]\n".length).trim();
+    if (body) sections.push({ title: "System", body });
+    return sections;
+  }
+
+  // Otherwise the head (up to the first [System] boundary, or the whole string)
+  // is the [Base] body.
+  const marker = "\n[System]\n";
+  const at = systemPrompt.indexOf(marker);
+  const head = at === -1 ? systemPrompt : systemPrompt.slice(0, at);
+  const baseBody = head.replace(/^\[Base]\n/, "").trim();
+  if (baseBody) sections.push({ title: "Base", body: baseBody });
+
+  if (at !== -1) {
+    const systemBody = systemPrompt.slice(at + marker.length).trim();
+    sections.push({ title: "System", body: systemBody });
+  }
+
+  return sections;
+}
+
 function parsePromptSections(text: string): PromptSection[] {
   const sections: PromptSection[] = [];
   let current: PromptSection | null = null;