digest: structural links, story-like prompt, agent file

When generating a digest, automatically link all source entries to the
digest node (journal entries → daily, dailies → weekly, weeklies →
monthly). This builds the temporal spine of the graph — previously
~4000 journal entries were disconnected islands unreachable by recall.

Rewrote digest prompt to produce narrative rather than reports:
capture the feel, the emotional arc, what it was like to live through
it. Letter to future self, not a task log.

Moved prompt to digest.agent file alongside other agent definitions.
Falls back to prompts/digest.md if agent file not found.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

poc-memory/src/agents/digest.rs

@@ -114,23 +114,34 @@ fn digest_node_key(level_name: &str, label: &str) -> String {
 
 // --- Input gathering ---
 
+/// Result of gathering inputs for a digest.
+struct GatherResult {
+    label: String,
+    /// (display_label, content) pairs for the prompt.
+    inputs: Vec<(String, String)>,
+    /// Store keys of source nodes — used to create structural links.
+    source_keys: Vec<String>,
+}
+
 /// Load child digest content from the store.
-fn load_child_digests(store: &Store, prefix: &str, labels: &[String]) -> Vec<(String, String)> {
+fn load_child_digests(store: &Store, prefix: &str, labels: &[String]) -> (Vec<(String, String)>, Vec<String>) {
     let mut digests = Vec::new();
+    let mut keys = Vec::new();
     for label in labels {
         let key = digest_node_key(prefix, label);
         if let Some(node) = store.nodes.get(&key) {
             digests.push((label.clone(), node.content.clone()));
+            keys.push(key);
         }
     }
-    digests
+    (digests, keys)
 }
 
 /// Unified: gather inputs for any digest level.
-fn gather(level: &DigestLevel, store: &Store, arg: &str) -> Result<(String, Vec<(String, String)>), String> {
+fn gather(level: &DigestLevel, store: &Store, arg: &str) -> Result<GatherResult, String> {
     let (label, dates) = (level.label_dates)(arg)?;
 
-    let inputs = if let Some(child_name) = level.child_name {
+    let (inputs, source_keys) = if let Some(child_name) = level.child_name {
         // Map parent's dates through child's date_to_label → child labels
         let child = LEVELS.iter()
             .find(|l| l.name == child_name)
@@ -143,19 +154,21 @@ fn gather(level: &DigestLevel, store: &Store, arg: &str) -> Result<(String, Vec<
         load_child_digests(store, child_name, &child_labels)
     } else {
         // Leaf level: scan store for episodic entries matching date
-        let mut entries: Vec<_> = store.nodes.values()
-            .filter(|n| n.node_type == store::NodeType::EpisodicSession
+        let mut entries: Vec<_> = store.nodes.iter()
+            .filter(|(_, n)| n.node_type == store::NodeType::EpisodicSession
                 && n.timestamp > 0
                 && store::format_date(n.timestamp) == label)
-            .map(|n| {
-                (store::format_datetime(n.timestamp), n.content.clone())
+            .map(|(key, n)| {
+                (store::format_datetime(n.timestamp), n.content.clone(), key.clone())
             })
             .collect();
         entries.sort_by(|a, b| a.0.cmp(&b.0));
-        entries
+        let keys = entries.iter().map(|(_, _, k)| k.clone()).collect();
+        let inputs = entries.into_iter().map(|(dt, c, _)| (dt, c)).collect();
+        (inputs, keys)
     };
 
-    Ok((label, inputs))
+    Ok(GatherResult { label, inputs, source_keys })
 }
 
 /// Unified: find candidate labels for auto-generation (past, not yet generated).
@@ -188,6 +201,7 @@ fn generate_digest(
     level: &DigestLevel,
     label: &str,
     inputs: &[(String, String)],
+    source_keys: &[String],
 ) -> Result<(), String> {
     println!("Generating {} digest for {}...", level.name, label);
 
@@ -209,15 +223,24 @@ fn generate_digest(
         .collect::<Vec<_>>()
         .join(", ");
 
-    let prompt = super::prompts::load_prompt("digest", &[
-        ("{{LEVEL}}", level.title),
-        ("{{PERIOD}}", level.period),
-        ("{{INPUT_TITLE}}", level.input_title),
-        ("{{LABEL}}", label),
-        ("{{CONTENT}}", &content),
-        ("{{COVERED}}", &covered),
-        ("{{KEYS}}", &keys_text),
-    ])?;
+    // Load prompt from agent file; fall back to prompts dir
+    let def = super::defs::get_def("digest");
+    let template = match &def {
+        Some(d) => d.prompt.clone(),
+        None => {
+            let path = crate::config::get().prompts_dir.join("digest.md");
+            std::fs::read_to_string(&path)
+                .map_err(|e| format!("load digest prompt: {}", e))?
+        }
+    };
+    let prompt = template
+        .replace("{{LEVEL}}", level.title)
+        .replace("{{PERIOD}}", level.period)
+        .replace("{{INPUT_TITLE}}", level.input_title)
+        .replace("{{LABEL}}", label)
+        .replace("{{CONTENT}}", &content)
+        .replace("{{COVERED}}", &covered)
+        .replace("{{KEYS}}", &keys_text);
     println!("  Prompt: {} chars (~{} tokens)", prompt.len(), prompt.len() / 4);
 
     println!("  Calling Sonnet...");
@@ -225,6 +248,32 @@ fn generate_digest(
 
     let key = digest_node_key(level.name, label);
     store.upsert_provenance(&key, &digest, "digest:write")?;
+
+    // Structural links: connect all source entries to this digest
+    let mut linked = 0;
+    for source_key in source_keys {
+        // Skip if link already exists
+        let exists = store.relations.iter().any(|r|
+            !r.deleted && r.source_key == *source_key && r.target_key == key);
+        if exists { continue; }
+
+        let source_uuid = store.nodes.get(source_key)
+            .map(|n| n.uuid).unwrap_or([0u8; 16]);
+        let target_uuid = store.nodes.get(&key)
+            .map(|n| n.uuid).unwrap_or([0u8; 16]);
+        let mut rel = new_relation(
+            source_uuid, target_uuid,
+            store::RelationType::Link, 0.8,
+            source_key, &key,
+        );
+        rel.provenance = "digest:structural".to_string();
+        store.add_relation(rel)?;
+        linked += 1;
+    }
+    if linked > 0 {
+        println!("  Linked {} source entries → {}", linked, key);
+    }
+
     store.save()?;
     println!("  Stored: {}", key);
 
@@ -238,8 +287,8 @@ pub fn generate(store: &mut Store, level_name: &str, arg: &str) -> Result<(), St
     let level = LEVELS.iter()
         .find(|l| l.name == level_name)
         .ok_or_else(|| format!("unknown digest level: {}", level_name))?;
-    let (label, inputs) = gather(level, store, arg)?;
-    generate_digest(store, level, &label, &inputs)
+    let result = gather(level, store, arg)?;
+    generate_digest(store, level, &result.label, &result.inputs, &result.source_keys)
 }
 
 // --- Auto-detect and generate missing digests ---
@@ -263,15 +312,15 @@ pub fn digest_auto(store: &mut Store) -> Result<(), String> {
         let mut skipped = 0u32;
 
         for arg in &candidates {
-            let (label, inputs) = gather(level, store, arg)?;
-            let key = digest_node_key(level.name, &label);
+            let result = gather(level, store, arg)?;
+            let key = digest_node_key(level.name, &result.label);
             if store.nodes.contains_key(&key) {
                 skipped += 1;
                 continue;
             }
-            if inputs.is_empty() { continue; }
-            println!("[auto] Missing {} digest for {}", level.name, label);
-            generate_digest(store, level, &label, &inputs)?;
+            if result.inputs.is_empty() { continue; }
+            println!("[auto] Missing {} digest for {}", level.name, result.label);
+            generate_digest(store, level, &result.label, &result.inputs, &result.source_keys)?;
             generated += 1;
         }