refactor(ai): read LLM settings from LlmConfig instead of env

Every LLM env reader now goes through ai::config::llm_config():
build_ollama / build_llamacpp / build_openrouter take config sections
(OPENROUTER_API_KEY is still read from env), and the backend switch,
embedding dim + prefixes, Ollama / llama.cpp / TTS timeouts, TTS voice and
ref length, agentic caps, chat turn timeout and the unified-search model
read their sections. LocalLlm::from_env -> from_config; the
test_daily_summary and populate_knowledge bins take Ollama defaults from
the config. env_max_iterations / env_default_num_ctx are renamed to
max_chat_iterations / default_num_ctx.

The picker still uses picker.include (= LLAMA_SWAP_ALLOWED_MODELS) until
discovery lands. Legacy env vars keep working through the config's
override layer; checked against the deployed .env, which resolves to the
same values. The LLM_BACKEND parsing tests move to ai::config.

llm.example.toml now leaves the embed prefixes empty: copying the
nomic prefixes onto a corpus embedded without them breaks search.

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

src/ai/tts.rs

@@ -169,13 +169,9 @@ fn sanitize_voice_name(raw: &str) -> Option<String> {
 
 /// Reference-clip cap in seconds for voice cloning. Chatterbox is zero-shot —
 /// a clean ~10–20s sample is the sweet spot and more rarely helps. Tune via
-/// `LLAMA_SWAP_TTS_REF_SECONDS` (default 30).
+/// `tts.ref_seconds` (default 30).
 fn tts_ref_seconds() -> u32 {
-    std::env::var("LLAMA_SWAP_TTS_REF_SECONDS")
-        .ok()
-        .and_then(|s| s.trim().parse::<u32>().ok())
-        .filter(|n| *n > 0)
-        .unwrap_or(30)
+    crate::ai::config::llm_config().tts.ref_seconds
 }
 
 /// Tag a (sanitized) voice name with the reference window used to create it:
@@ -228,12 +224,9 @@ fn resolve_ref_window(
 }
 
 /// Optional default voice for synthesis when the request doesn't name one.
-/// Set `LLAMA_SWAP_TTS_VOICE=m` to read insights in a cloned voice by default.
+/// Set `tts.voice = "m"` to read insights in a cloned voice by default.
 fn default_voice() -> Option<String> {
-    std::env::var("LLAMA_SWAP_TTS_VOICE")
-        .ok()
-        .map(|s| s.trim().to_string())
-        .filter(|s| !s.is_empty())
+    crate::ai::config::llm_config().tts.voice.clone()
 }
 
 // Markdown / formatting strippers, compiled once. Insight text is markdown,