tools/memory: one function per tool

Split the monolithic dispatch(name, args) into individual public
functions (render, write, search, links, link_set, link_add, used,
weight_set, rename, supersede, query, output, journal_tail,
journal_new, journal_update) each with a matching _def() function.

The old dispatch() remains as a thin match for backward compat
until the Tool registry replaces it.

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

src/agent/tools/mod.rs

@@ -21,10 +21,25 @@ mod vision;
 pub mod working_stack;
 
 use serde::{Serialize, Deserialize};
+use std::future::Future;
+use std::pin::Pin;
 use std::time::Instant;
 
 fn default_timeout() -> u64 { 120 }
 
+/// Async tool handler function.
+/// Agent is None when called from contexts without an agent (MCP server, subconscious).
+pub type ToolHandler = fn(
+    Option<std::sync::Arc<tokio::sync::Mutex<super::Agent>>>,
+    serde_json::Value,
+) -> Pin<Box<dyn Future<Output = anyhow::Result<String>> + Send>>;
+
+/// A tool with its definition and handler — single source of truth.
+pub struct Tool {
+    pub def: ToolDef,
+    pub handler: ToolHandler,
+}
+
 /// Function call within a tool call — name + JSON arguments.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct FunctionCall {