toolsrv: auto-load tools on first use

When an agent calls a tool that exists but isn't loaded, toolsrv now
automatically loads it instead of returning an error. This eliminates
the round-trip of a separate load call.

- Remove Registry.Exists() (superseded by auto-load logic)
- Fire OnToolsChanged callback after auto-load so olliesrv can update
  tool definitions for subsequent turns

cmd/toolsrv/internal/server/proc.go

@@ -275,7 +275,14 @@ func (st *State) NewProc(ctx context.Context, payload string, background bool) (
 
 	info, ok := reg.Lookup(aid, toolName)
 	if !ok {
-		return "", 0, fmt.Errorf("tool not found: %s", toolName)
+		// Auto-load if tool exists
+		if err := reg.Load(aid, toolName); err != nil {
+			return "", 0, err // "tool not found: X"
+		}
+		info, _ = reg.Lookup(aid, toolName)
+		if st.OnToolsChanged != nil {
+			st.OnToolsChanged()
+		}
 	}
 
 	// Create proc context. For background procs, detach from request context