tools: unify channel and memory tools, clean up mcp-server

Move all tool definitions and dispatch out of mcp-server.rs:
- Channel tools: new tools/channels.rs with definitions, async
  dispatch, blocking dispatch, and capnp RPC helpers
- Memory tools: make tools/memory.rs pub so mcp-server can use it

mcp-server.rs is now pure JSON-RPC protocol plumbing (482 → 169 lines).
No tool-specific code remains in that file.

Also removes duplicated channel RPC helpers and fetch_all_channels
that were in both mcp-server.rs and thalamus/channels.rs.

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

src/agent/tools/mod.rs

@@ -6,10 +6,11 @@
 
 // Core tools
 mod bash;
+pub mod channels;
 mod edit;
 mod glob;
 mod grep;
-mod memory;
+pub mod memory;
 mod read;
 mod web;
 mod write;
@@ -191,6 +192,15 @@ pub async fn dispatch_shared(
         });
     }
 
+    // Channel tools
+    if name.starts_with("channel_") {
+        let result = channels::dispatch(name, args).await;
+        return Some(match result {
+            Ok(s) => ToolOutput::text(s),
+            Err(e) => ToolOutput::error(e),
+        });
+    }
+
     // File and execution tools
     let result = match name {
         "read_file" => read::read_file(args),
@@ -225,6 +235,7 @@ pub fn definitions() -> Vec<ToolDef> {
         glob::definition(),
     ];
     defs.extend(control::definitions());
+    defs.extend(channels::definitions());
     defs.extend(memory::definitions());
     defs
 }