tools: add memory_query for structured graph queries

Exposes the full query language as a tool: filtering, sorting, field
selection, neighbor walks. Examples:
  degree > 10 | sort weight | limit 5
  neighbors('identity') | select strength
  key ~ 'journal.*' | count

Also added query_to_string() in the parser so queries return strings
instead of printing to stdout. Updated memory-instructions-core to
list all current tools (added memory_query and journal, removed
CLI commands section and nonexistent memory_search_content).

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/query/parser.rs

@@ -520,6 +520,51 @@ pub fn run_query(store: &Store, graph: &Graph, query_str: &str) -> Result<(), St
     Ok(())
 }
 
+/// Run a query and return the output as a string (for tool calls).
+pub fn query_to_string(store: &Store, graph: &Graph, query_str: &str) -> Result<String, String> {
+    let q = query_parser::query(query_str)
+        .map_err(|e| format!("Parse error: {}", e))?;
+
+    let results = execute_parsed(store, graph, &q)?;
+
+    if q.stages.iter().any(|s| matches!(s, Stage::Count)) {
+        return Ok(results.len().to_string());
+    }
+    if results.is_empty() {
+        return Ok("no results".to_string());
+    }
+
+    let fields: Option<&Vec<String>> = q.stages.iter().find_map(|s| match s {
+        Stage::Select(f) => Some(f),
+        _ => None,
+    });
+
+    let mut out = String::new();
+    if let Some(fields) = fields {
+        let mut header = vec!["key".to_string()];
+        header.extend(fields.iter().cloned());
+        out.push_str(&header.join("\t"));
+        out.push('\n');
+        for r in &results {
+            let mut row = vec![r.key.clone()];
+            for f in fields {
+                row.push(match r.fields.get(f) {
+                    Some(v) => format_value(v),
+                    None => "-".to_string(),
+                });
+            }
+            out.push_str(&row.join("\t"));
+            out.push('\n');
+        }
+    } else {
+        for r in &results {
+            out.push_str(&r.key);
+            out.push('\n');
+        }
+    }
+    Ok(out)
+}
+
 // -- Connectivity analysis --
 
 /// BFS shortest path between two nodes, max_hops limit.