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/agent/tools/memory.rs

@@ -38,6 +38,11 @@ pub fn definitions() -> Vec<ToolDef> {
         ToolDef::new("memory_supersede",
             "Mark a node as superseded by another (sets weight to 0.01).",
             json!({"type":"object","properties":{"old_key":{"type":"string"},"new_key":{"type":"string"},"reason":{"type":"string"}},"required":["old_key","new_key"]})),
+        ToolDef::new("memory_query",
+            "Run a structured query against the memory graph. Supports filtering, \
+             sorting, field selection. Examples: \"degree > 10 | sort weight | limit 5\", \
+             \"neighbors('identity') | select strength\", \"key ~ 'journal.*' | count\"",
+            json!({"type":"object","properties":{"query":{"type":"string","description":"Query expression"}},"required":["query"]})),
     ]
 }
 
@@ -102,6 +107,13 @@ pub fn dispatch(name: &str, args: &serde_json::Value, provenance: Option<&str>)
             store.save().map_err(|e| anyhow::anyhow!("{}", e))?;
             Ok(format!("superseded {} → {} ({})", old_key, new_key, reason))
         }
+        "memory_query" => {
+            let query = get_str(args, "query")?;
+            let store = Store::load().map_err(|e| anyhow::anyhow!("{}", e))?;
+            let graph = store.build_graph();
+            crate::query_parser::query_to_string(&store, &graph, query)
+                .map_err(|e| anyhow::anyhow!("{}", e))
+        }
         _ => anyhow::bail!("Unknown memory tool: {}", name),
     }
 }