docs: document llm.toml and llama-swap metadata discovery

README, CLAUDE.md and .env.example now point at llm.toml for LLM settings
(OPENROUTER_API_KEY stays in env) and describe the legacy env override
window, the discovered picker and the startup cross-check. ADR-0003 records
the move and amends ADR-0002's deployment-config home; CONTEXT.md's
Deployment config / Hard default entries follow. Source comments and the
"set LLAMA_SWAP_URL" error strings name the llm.toml keys.

Also fixes README's AGENTIC_MAX_ITERATIONS default (10 -> 12, what the
code uses).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/unified_search.rs

@@ -635,7 +635,7 @@ async fn translate<TagD: TagDao>(
         }
     };
 
-    // Respect env/config for the LLM backend (LLM_BACKEND → ollama or
+    // Respect env/config for the LLM backend (llm.toml `backend` → ollama or
     // llama-swap); local only, no hybrid, per the feature's design.
     //
     // Translation-model precedence: