llm: full per-agent usage logging with prompts and responses

Log every model call to ~/.claude/memory/llm-logs/YYYY-MM-DD.md with
full prompt, response, agent type, model, duration, and status. One
file per day, markdown formatted for easy reading.

Agent types: fact-mine, experience-mine, consolidate, knowledge,
digest, enrich, audit. This gives visibility into what each agent
is doing and whether to adjust prompts or frequency.

src/audit.rs

@@ -211,7 +211,7 @@ pub fn link_audit(store: &mut Store, apply: bool) -> Result<AuditStats, String>
     // Run batches in parallel via rayon
     let batch_results: Vec<_> = batch_data.par_iter()
         .map(|(batch_idx, batch_infos, prompt)| {
-            let response = call_sonnet(prompt, 300);
+            let response = call_sonnet("audit", prompt);
             let completed = done.fetch_add(1, Ordering::Relaxed) + 1;
             eprint!("\r  Batches: {}/{} done", completed, total_batches);
             (*batch_idx, batch_infos, response)