cleanup: bump qwen2.5 → qwen3.5:latest in active defaults

stronger local rung is now the small-model-pipeline tier-1 default
across both Rust legacy + Go rewrite (cf. golangLAKEHOUSE phase 1).
same JSON-clean property as qwen2.5, more capacity. ollama still
serves both side-by-side; rollback is a 4-line revert if a workload
regresses.

active-default sites:
- lakehouse.toml [ai] gen_model + rerank_model → qwen3.5:latest
- mcp-server/observer.ts diagnose call (Phase 44 /v1/chat path) → qwen3.5:latest
- mcp-server/index.ts model roster doc → qwen3.5:latest first
- crates/vectord/src/rag.rs ContinuableOpts + RagResponse.model → qwen3.5:latest

skipped: execution_loop/mod.rs comments describing historic qwen2.5
tool_call quirks — those are documentation of past behavior, not
active defaults. data/_catalog/profiles/*.json are runtime-generated
(gitignored), not in scope for tracked changes.

cargo check -p vectord: clean. no behavioral change in the audit
pipeline — same JSON-clean local model, same think=Some(false)
posture, just stronger upstream.

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

mcp-server/index.ts

@@ -313,9 +313,9 @@ ${(buckets as any[] || []).map((b: any) => `- ${b.name}: ${b.backend} (${b.reach
 - Ollama: :11434
 
 ## Available Models
+- qwen3.5:latest: stronger local rung, JSON-clean (default for gen + rerank)
 - qwen3: 8.2B, 40K context, thinking+tools (best for reasoning)
-- qwen2.5: 7B, 8K context (best for fast SQL generation)
-- mistral: 7B, 8K context (general generation)
+- qwen2.5: 7B, 8K context (legacy — 2026-04 era comparison runs only)
 - nomic-embed-text: 137M (embedding, automatic)
 `;
   return { contents: [{ uri: uri.href, mimeType: "text/plain", text }] };