feat(acp/handlers): fs/read_text_file + fs/write_text_file with path safety

internal/acp/handlers/server_fs_write.go

@@ -0,0 +1,40 @@
+package handlers
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+// FsWriteHandler handles fs/write_text_file (spec §5.4).
+type FsWriteHandler struct{}
+
+// NewFsWriteHandler constructs an FsWriteHandler.
+func NewFsWriteHandler() *FsWriteHandler { return &FsWriteHandler{} }
+
+type fsWriteParams struct {
+	SessionID string `json:"sessionId"`
+	Path      string `json:"path"`
+	Content   string `json:"content"`
+}
+
+// Handle writes a file inside sess.CwdPath, creating parent dirs as needed.
+// Returns FsError for path-escape or filesystem failure.
+func (h *FsWriteHandler) Handle(_ context.Context, sess SessionContext, paramsJSON json.RawMessage) FsResult {
+	var p fsWriteParams
+	if err := json.Unmarshal(paramsJSON, &p); err != nil {
+		return FsResult{Err: &FsError{Code: -32602, Message: "invalid params: " + err.Error()}}
+	}
+	abs, err := resolveSafePath(sess.CwdPath, p.Path)
+	if err != nil {
+		return FsResult{Err: &FsError{Code: -32001, Message: err.Error()}}
+	}
+	if err := os.MkdirAll(filepath.Dir(abs), 0o755); err != nil {
+		return FsResult{Err: &FsError{Code: -32603, Message: "mkdir: " + err.Error()}}
+	}
+	if err := os.WriteFile(abs, []byte(p.Content), 0o644); err != nil {
+		return FsResult{Err: &FsError{Code: -32603, Message: "write: " + err.Error()}}
+	}
+	return FsResult{OK: json.RawMessage(`{}`)}
+}