observer: overseer fallback label → qwen3.5:latest (matches reverted route)

Mirror of yesterday's execution_loop overseer revert (commit d054c0b).
The observer logs an "overseer:<model>" endpoint string for analysis;
when row.model is missing it falls back to a hardcoded label. PR #13
set that fallback to "claude-opus-4-7" — but the route now goes to
local Ollama qwen3.5:latest, so the label was wrong.

Trivial one-line fix, no behavior change. Just keeps observer's
endpoint string honest when older rows from the cloud-routing window
get re-analyzed.

End-to-end verification of the local hot path (post-revert):
  BEFORE  /v1/usage by_provider: []
  AFTER   /v1/usage by_provider: [{"k":"ollama","v":2}]
  → /v1/iterate fired 2 chat calls, both to local ollama
  → ZERO cloud requests (no kimi/openrouter/opencode/ollama_cloud)
  → API meter on cloud providers stays at 0 for customer requests

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

mcp-server/observer.ts

@@ -771,7 +771,7 @@ async function tailOverseerCorrections(): Promise<number> {
     try { row = JSON.parse(line); } catch { continue; }
     const op: ObservedOp = {
       timestamp: row.created_at ?? new Date().toISOString(),
-      endpoint: `overseer:${row.model ?? "claude-opus-4-7"}`,
+      endpoint: `overseer:${row.model ?? "qwen3.5:latest"}`,
       input_summary: `${row.task_class ?? "?"}: ${row.reason ?? "escalation"}`,
       // Correction itself is neither success nor failure — it's a
       // mitigation attempt. We mark success=true so analyzeErrors