experience-mine: transcript-level dedup via content hash

Running the miner twice on the same transcript produced near-duplicate
entries because:
1. Prompt-based dedup (passing recent entries to Sonnet) doesn't catch
   semantic duplicates written in a different emotional register
2. Key-based dedup (timestamp + content slug) fails because Sonnet
   assigns different timestamps and wording each run

Fix: hash the transcript file content before mining. Store the hash
as a _mined-transcripts node. Skip if already present.

Limitation: doesn't catch overlapping content when a live transcript
grows between runs (content hash changes). This is fine — the miner
is intended for archived conversations, not live ones.

Tested: second run on same transcript correctly skipped with
"Already mined this transcript" message.

src/digest.rs

@@ -12,7 +12,9 @@ use crate::capnp_store::{self, Store};
 use crate::neuro;
 
 use regex::Regex;
+use std::collections::hash_map::DefaultHasher;
 use std::fs;
+use std::hash::{Hash, Hasher};
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
@@ -1777,6 +1779,20 @@ pub fn experience_mine(
     jsonl_path: &str,
 ) -> Result<usize, String> {
     println!("Experience mining: {}", jsonl_path);
+
+    // Transcript-level dedup: hash the file content and check if already mined
+    let transcript_bytes = fs::read(jsonl_path)
+        .map_err(|e| format!("reading transcript: {}", e))?;
+    let mut hasher = DefaultHasher::new();
+    transcript_bytes.hash(&mut hasher);
+    let hash = hasher.finish();
+    let dedup_key = format!("_mined-transcripts.md#h-{:016x}", hash);
+
+    if store.nodes.contains_key(&dedup_key) {
+        println!("  Already mined this transcript ({}), skipping.", &dedup_key[24..]);
+        return Ok(0);
+    }
+
     let messages = extract_conversation(jsonl_path)?;
     let conversation = format_conversation(&messages);
     println!("  {} messages, {} chars", messages.len(), conversation.len());
@@ -1868,9 +1884,16 @@ pub fn experience_mine(
         println!("  + [{}] {}...", ts, preview);
     }
 
+    // Record this transcript as mined (even if count == 0, to prevent re-runs)
+    let dedup_content = format!("Mined {} ({} entries)", jsonl_path, count);
+    let mut dedup_node = Store::new_node(&dedup_key, &dedup_content);
+    dedup_node.category = capnp_store::Category::Task;
+    let _ = store.upsert_node(dedup_node);
+
     if count > 0 {
-        store.save()?;
         println!("  Saved {} new journal entries.", count);
     }
+    store.save()?;
+    println!("Done: {} new entries mined.", count);
     Ok(count)
 }