v1.13.6: compaction head-assembly audit + reasoning fix

Audit traced compaction's summary path post-v1.13.1-B read flip:
- Q1: reads from messages_with_parts (view) — clean
- Q2: parts shape correctly threaded through buildHeadPayload — clean
- Q3: reasoning omitted from summary input — FIX NEEDED

v1.13.1-C wired reasoning end-to-end into inference/payload.ts but
missed this read site. Summarizer model couldn't see the reasoning
trail for tool-bearing turns, quietly degrading summary quality for
reasoning-channel models (qwen3.6).

Fix:
- CompactionMessage extended with reasoning_parts field
- SELECT pulls reasoning_parts from messages_with_parts
- buildHeadPayload (now exported for tests) prefixes assistant content
  with <reasoning>...</reasoning>\n\n<content>... when reasoning is
  present; standalone <reasoning>...</reasoning> for tool-call-only
  turns; omits the tag when reasoning is null or empty

4 new render branch tests (190 total).

Smoke deferred: forcing real compaction requires either threshold
pollution or building up a >40k-token chat with reasoning_parts.
Render branches are unit-covered; integration would only re-prove
structural correctness.

apps/server/src/services/compaction.ts

@@ -39,6 +39,11 @@ export interface CompactionMessage {
   status: 'streaming' | 'complete' | 'failed' | 'cancelled';
   tool_calls: Array<{ id: string; name: string; args: Record<string, unknown> }> | null;
   tool_results: { tool_call_id: string; output: unknown; truncated: boolean; error?: string } | null;
+  // v1.13.6: reasoning_parts captured by v1.13.1-C and read back through
+  // messages_with_parts. Embedded into the head-assembly payload as prose so
+  // the summarizer LLM sees what the model was reasoning through when it
+  // chose its tool calls.
+  reasoning_parts: Array<{ text: string }> | null;
   metadata: { kind?: string } | null;
   created_at: string;
 }
@@ -197,7 +202,8 @@ export function buildPrompt(
 // would silently drop pre-legacy-compact history before the LLM sees it.
 // Compaction wants to send the entire head, full stop.) ===
 
-interface OpenAiMessage {
+// v1.13.6: exported for unit-test access (reasoning render coverage).
+export interface OpenAiMessage {
   role: 'system' | 'user' | 'assistant' | 'tool';
   content: string | null;
   tool_calls?: Array<{
@@ -212,7 +218,8 @@ function isCapHitSentinel(m: CompactionMessage): boolean {
   return m.role === 'system' && m.metadata != null && m.metadata.kind === 'cap_hit';
 }
 
-function buildHeadPayload(head: CompactionMessage[]): OpenAiMessage[] {
+// v1.13.6: exported for unit-test access (reasoning render coverage).
+export function buildHeadPayload(head: CompactionMessage[]): OpenAiMessage[] {
   const out: OpenAiMessage[] = [];
   for (const m of head) {
     if (isCapHitSentinel(m)) continue;
@@ -243,9 +250,22 @@ function buildHeadPayload(head: CompactionMessage[]): OpenAiMessage[] {
       continue;
     }
     if (m.role === 'assistant') {
+      // v1.13.6: embed reasoning text as prose prefixed onto the assistant
+      // content. OpenAI wire shape doesn't carry reasoning as a structured
+      // field, but the summarizer is reading text — a tagged prose block
+      // gives it the same signal. We mirror the AI SDK ReasoningPart shape
+      // by using a <reasoning>...</reasoning> wrapper so the summarizer can
+      // distinguish reasoning from user-visible answer.
+      let body = m.content && m.content.length > 0 ? m.content : '';
+      if (m.reasoning_parts && m.reasoning_parts.length > 0) {
+        const reasoning = m.reasoning_parts.map((r) => r.text).join('');
+        body = body.length > 0
+          ? `<reasoning>${reasoning}</reasoning>\n\n${body}`
+          : `<reasoning>${reasoning}</reasoning>`;
+      }
       const msg: OpenAiMessage = {
         role: 'assistant',
-        content: m.content && m.content.length > 0 ? m.content : null,
+        content: body.length > 0 ? body : null,
       };
       if (m.tool_calls && m.tool_calls.length > 0) {
         msg.tool_calls = m.tool_calls.map((tc) => ({
@@ -344,8 +364,11 @@ export async function process(input: ProcessInput): Promise<void> {
   // turns() boundary logic sees the same sequence the LLM will.
   // v1.13.1-B: reads tool_calls/tool_results via the parts-merged view so
   // the compaction payload matches what the LLM saw on the original turn.
+  // v1.13.6: also pulls reasoning_parts (added in v1.13.1-C) so summaries
+  // capture what the model was working through before each tool call.
   const messages = await sql<CompactionMessage[]>`
-    SELECT id, role, content, kind, summary, status, tool_calls, tool_results, metadata, created_at
+    SELECT id, role, content, kind, summary, status, tool_calls, tool_results,
+           reasoning_parts, metadata, created_at
     FROM messages_with_parts
     WHERE chat_id = ${chatId} AND compacted_at IS NULL
     ORDER BY created_at ASC, id ASC