feat(plugins): implement yaegi plugin loader with basic test

Move yaegi_symbols to pkg/yaegi_symbols/ to avoid import cycle.
Create pkg/plugins/yaegi/ package with LoadPlugin function.
The basic Plugin interface assertion (Name, Version) passes.

pkg/plugins/yaegi/loader.go

@@ -0,0 +1,70 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package yaegi
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"code.vikunja.io/api/pkg/plugins"
+	"code.vikunja.io/api/pkg/yaegi_symbols"
+
+	"github.com/traefik/yaegi/interp"
+	"github.com/traefik/yaegi/stdlib"
+)
+
+// LoadPlugin loads a plugin from a directory of Go source files using the Yaegi interpreter.
+func LoadPlugin(dir string) (plugins.Plugin, error) {
+	i := interp.New(interp.Options{})
+	i.Use(stdlib.Symbols)
+	i.Use(yaegi_symbols.Symbols)
+
+	// Read all .go files in the directory
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return nil, fmt.Errorf("reading plugin dir %s: %w", dir, err)
+	}
+
+	for _, e := range entries {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".go") {
+			continue
+		}
+		src, err := os.ReadFile(filepath.Join(dir, e.Name()))
+		if err != nil {
+			return nil, fmt.Errorf("reading %s: %w", e.Name(), err)
+		}
+		_, err = i.Eval(string(src))
+		if err != nil {
+			return nil, fmt.Errorf("evaluating %s: %w", e.Name(), err)
+		}
+	}
+
+	// Look up the NewPlugin entry point
+	v, err := i.Eval("main.NewPlugin")
+	if err != nil {
+		return nil, fmt.Errorf("looking up NewPlugin: %w", err)
+	}
+
+	newPlugin, ok := v.Interface().(func() plugins.Plugin)
+	if !ok {
+		return nil, fmt.Errorf("NewPlugin has wrong signature: %T", v.Interface())
+	}
+
+	return newPlugin(), nil
+}

pkg/plugins/yaegi/loader_test.go

@@ -0,0 +1,44 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package yaegi
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadPlugin(t *testing.T) {
+	pluginDir := filepath.Join("..", "..", "..", "examples", "plugins", "example")
+
+	mainGo := filepath.Join(pluginDir, "main.go")
+	if _, err := os.Stat(mainGo); err != nil {
+		t.Fatalf("example plugin source not found at %s: %v", mainGo, err)
+	}
+
+	p, err := LoadPlugin(pluginDir)
+	if err != nil {
+		t.Fatalf("LoadPlugin failed: %v", err)
+	}
+
+	if p.Name() != "example" {
+		t.Errorf("expected plugin name 'example', got %q", p.Name())
+	}
+	if p.Version() != "1.0.0" {
+		t.Errorf("expected version '1.0.0', got %q", p.Version())
+	}
+}