docs: May 2026 tool-calling update + agentic retrieval landscape

Two new corpus docs plus targeted GOTCHAS corrections based on
Ollama/llama.cpp/vLLM issue tracker review, late-2025/early-2026
research papers, and convergent empirical evidence from two production
Gemma 4 agents.

New:
- CORPUS_tool_calling_2026-05.md: current state of tool calling.
  Properly characterizes the think flag (parser-side bug, not
  model-side; recipe by loop shape), surfaces ollama/ollama#15539,
  documents the "doesn't fire tools without explicit ask" research
  (Probe & Prefill, BiasBusters, When2Call), XGrammar-2.
- CORPUS_agentic_retrieval.md: post-RAG landscape. Hybrid retrieval
  + cross-encoder rerank as the settled base, CRAG-shaped flows,
  LightRAG/LazyGraphRAG, mem0 vs Letta MemFS, deep-research agents.
  Production-vs-experimental sorting for self-hosted small-model use.

Updates:
- GOTCHAS.md: the think:false rule, Vulkan unused-token loop, and
  Ollama 0.20 streaming bug all marked SUPERSEDED/FIXED/PARTIALLY
  FIXED in-place with pointers to the May update.
- README.md: indexed the two new docs.
- .gitignore: added private/ for bot-specific notes.

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

.gitignore

@@ -13,3 +13,6 @@ scripts/bakeoff/runs*/*/work/
 # Editor / OS
 .DS_Store
 *.swp
+
+# Private — bot-specific findings, recommendations tied to non-public projects
+private/