feat: ConfigRegistry + SettingsAPI for plugin config access

Core exposes a unified settings interface through Plugin SDK:
- internal/config/registry.go: thread-safe namespaced KV store
  - Register/Get/Set/List/Delete with JSON persistence
  - Flush() for explicit save to disk
- SettingsAPI in SDK: plugins call plugin.Settings().Get/Set/List
  - Full access: read/write any key (core.*, plugin.<name>.*)
- plugin.Registry.SetConfigRegistry() wires it into SDK plugins
- main.go registers core.* keys at startup, flushes on shutdown
- 7 tests for ConfigRegistry

internal/config/registry.go

@@ -0,0 +1,133 @@
+package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
+)
+
+type ConfigRegistry struct {
+	mu       sync.RWMutex
+	values   map[string]interface{}
+	persistPath string
+	dirty    bool
+}
+
+func NewConfigRegistry(persistPath string) *ConfigRegistry {
+	r := &ConfigRegistry{
+		values:      make(map[string]interface{}),
+		persistPath: persistPath,
+	}
+	r.load()
+	return r
+}
+
+func (r *ConfigRegistry) Register(key string, value interface{}) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if _, exists := r.values[key]; !exists {
+		r.values[key] = value
+	}
+}
+
+func (r *ConfigRegistry) RegisterDefault(key string, value interface{}) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if _, exists := r.values[key]; !exists {
+		r.values[key] = value
+	}
+}
+
+func (r *ConfigRegistry) Get(key string) (interface{}, error) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	v, ok := r.values[key]
+	if !ok {
+		return nil, fmt.Errorf("config key %q not found", key)
+	}
+	return v, nil
+}
+
+func (r *ConfigRegistry) Set(key string, value interface{}) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.values[key] = value
+	r.dirty = true
+	return nil
+}
+
+func (r *ConfigRegistry) List(prefix string) []string {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	var keys []string
+	for k := range r.values {
+		if prefix == "" || strings.HasPrefix(k, prefix) {
+			keys = append(keys, k)
+		}
+	}
+	sort.Strings(keys)
+	return keys
+}
+
+func (r *ConfigRegistry) Delete(key string) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	delete(r.values, key)
+	r.dirty = true
+	return nil
+}
+
+func (r *ConfigRegistry) Dump() map[string]interface{} {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	cp := make(map[string]interface{})
+	for k, v := range r.values {
+		cp[k] = v
+	}
+	return cp
+}
+
+func (r *ConfigRegistry) Flush() error {
+	r.mu.RLock()
+	if !r.dirty {
+		r.mu.RUnlock()
+		return nil
+	}
+	r.mu.RUnlock()
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if r.persistPath == "" {
+		return nil
+	}
+	os.MkdirAll(filepath.Dir(r.persistPath), 0755)
+	data, err := json.MarshalIndent(r.values, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal config: %w", err)
+	}
+	if err := os.WriteFile(r.persistPath, data, 0644); err != nil {
+		return fmt.Errorf("write config: %w", err)
+	}
+	r.dirty = false
+	return nil
+}
+
+func (r *ConfigRegistry) load() {
+	if r.persistPath == "" {
+		return
+	}
+	data, err := os.ReadFile(r.persistPath)
+	if err != nil {
+		return
+	}
+	var vals map[string]interface{}
+	if err := json.Unmarshal(data, &vals); err != nil {
+		return
+	}
+	r.values = vals
+}