feat: import agents_and_robots platform as unibots (Matrix-out, unibus transport)

Reemplaza el scaffold del echobot por la plataforma completa de bots traida
desde ~/DataProyects/Github/agents_and_robots tras la operacion Matrix-out:
los bots ya no hablan por Matrix sino por el bus unibus (modelo todo-rooms +
E2E via shell/transportunibus sobre github.com/enmanuel/unibus/pkg/client).

- go.mod: replace de unibus -> ../unibus y de fn-registry -> ../../../.. (paths
  relativos reajustados a la nueva ubicacion dentro de fn_registry).
- app.md: bump a 0.2.0, descripcion + arquitectura + comandos + gotchas reales.
- modulo Go conservado como github.com/enmanuel/agents (sin reescribir imports).

agents_and_robots queda archivado como museo de la era Matrix.

tools/file/append.go

@@ -0,0 +1,83 @@
+package file
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/enmanuel/agents/internal/config"
+	"github.com/enmanuel/agents/tools"
+)
+
+// maxAppendTotal is the maximum total file size after appending (10 MB).
+const maxAppendTotal = 10 * 1024 * 1024
+
+// NewAppendFile creates an append_file tool that appends content to a local file.
+// Deny-by-default: if AllowedPaths is empty, all operations are rejected.
+// Rejects if ReadOnly is true. Creates the file (and parent directories) if it does not exist.
+func NewAppendFile(cfg config.FileOpsCfg) tools.Tool {
+	return tools.Tool{
+		Def: tools.Def{
+			Name:        "append_file",
+			Description: "Append content to the end of a local file. Creates the file if it does not exist.",
+			Parameters: []tools.Param{
+				{Name: "path", Type: "string", Description: "Absolute path to the file to append to", Required: true},
+				{Name: "content", Type: "string", Description: "Content to append to the file", Required: true},
+			},
+		},
+		Exec: func(ctx context.Context, args map[string]any) tools.Result {
+			path := tools.GetString(args, "path")
+			if path == "" {
+				return tools.Result{Err: fmt.Errorf("append_file: path is required")}
+			}
+
+			content := tools.GetString(args, "content")
+			if content == "" {
+				return tools.Result{Err: fmt.Errorf("append_file: content is required")}
+			}
+
+			absPath, err := filepath.Abs(path)
+			if err != nil {
+				return tools.Result{Err: fmt.Errorf("append_file: %w", err)}
+			}
+
+			if err := validateWritePath(absPath, cfg.AllowedPaths, cfg.ReadOnly); err != nil {
+				return tools.Result{Err: err}
+			}
+
+			// Check existing file size to enforce the total limit.
+			var existingSize int64
+			info, err := os.Stat(absPath)
+			if err == nil {
+				existingSize = info.Size()
+			}
+			// err != nil means file doesn't exist, which is fine (will be created).
+
+			newTotal := existingSize + int64(len(content))
+			if newTotal > maxAppendTotal {
+				return tools.Result{Err: fmt.Errorf("append_file: resulting file size (%d bytes) exceeds 10 MB limit", newTotal)}
+			}
+
+			// Create parent directories if they don't exist.
+			dir := filepath.Dir(absPath)
+			if err := os.MkdirAll(dir, 0755); err != nil {
+				return tools.Result{Err: fmt.Errorf("append_file: cannot create directories: %w", err)}
+			}
+
+			f, err := os.OpenFile(absPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+			if err != nil {
+				return tools.Result{Err: fmt.Errorf("append_file: %w", err)}
+			}
+			defer f.Close()
+
+			n, err := f.WriteString(content)
+			if err != nil {
+				return tools.Result{Err: fmt.Errorf("append_file: %w", err)}
+			}
+
+			finalSize := existingSize + int64(n)
+			return tools.Result{Output: fmt.Sprintf("appended %d bytes to %s (total size: %d bytes)", n, absPath, finalSize)}
+		},
+	}
+}