Add config-based close hooks (bd-g4b4)

Implements on_close hooks in .beads/config.yaml for automation and
notifications. Hooks receive issue data via environment variables
(BEAD_ID, BEAD_TITLE, BEAD_TYPE, BEAD_PRIORITY, BEAD_CLOSE_REASON)
and run via sh -c.

Changes:
- internal/config: Add HookEntry type and GetCloseHooks()
- internal/hooks: Add RunConfigCloseHooks() for executing config hooks
- cmd/bd: Call RunConfigCloseHooks after successful close
- docs/CONFIG.md: Document hooks configuration with examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/config.go

@@ -306,6 +306,43 @@ func ResolveExternalProjectPath(projectName string) string {
 	return path
 }
 
+// HookEntry represents a single config-based hook
+type HookEntry struct {
+	Command string `yaml:"command" mapstructure:"command"` // Shell command to run
+	Name    string `yaml:"name" mapstructure:"name"`       // Optional display name
+}
+
+// GetCloseHooks returns the on_close hooks from config
+func GetCloseHooks() []HookEntry {
+	if v == nil {
+		return nil
+	}
+	var hooks []HookEntry
+	raw := v.Get("hooks.on_close")
+	if raw == nil {
+		return nil
+	}
+
+	// Handle slice of maps (from YAML parsing)
+	if rawSlice, ok := raw.([]interface{}); ok {
+		for _, item := range rawSlice {
+			if m, ok := item.(map[string]interface{}); ok {
+				entry := HookEntry{}
+				if cmd, ok := m["command"].(string); ok {
+					entry.Command = cmd
+				}
+				if name, ok := m["name"].(string); ok {
+					entry.Name = name
+				}
+				if entry.Command != "" {
+					hooks = append(hooks, entry)
+				}
+			}
+		}
+	}
+	return hooks
+}
+
 // GetIdentity resolves the user's identity for messaging.
 // Priority chain:
 //  1. flagValue (if non-empty, from --identity flag)