Remove experience_mine, journal_enrich, and old mining helpers

experience_mine and journal_enrich are replaced by the observation
agent. enrich.rs reduced from 465 to 40 lines — only extract_conversation
and split_on_compaction remain (used by observation fragment selection).

-455 lines.

poc-memory/src/main.rs

@@ -1017,21 +1017,8 @@ fn cmd_digest(level: DigestLevel) -> Result<(), String> {
     }
 }
 
-fn cmd_experience_mine(jsonl_path: Option<String>) -> Result<(), String> {
-    let jsonl_path = match jsonl_path {
-        Some(p) => p,
-        None => cli::journal::find_current_transcript()
-            .ok_or("no JSONL transcripts found")?,
-    };
-
-    if !std::path::Path::new(jsonl_path.as_str()).is_file() {
-        return Err(format!("JSONL not found: {}", jsonl_path));
-    }
-
-    let mut store = store::Store::load()?;
-    let count = crate::enrich::experience_mine(&mut store, &jsonl_path, None)?;
-    println!("Done: {} new entries mined.", count);
-    Ok(())
+fn cmd_experience_mine(_jsonl_path: Option<String>) -> Result<(), String> {
+    Err("experience-mine has been removed — use the observation agent instead.".into())
 }
 
 fn cmd_daemon(sub: DaemonCmd) -> Result<(), String> {