feat(types): add custom type support to beads config (bd-649s)

Add types.custom config key mirroring the status.custom pattern:
- Add CustomTypeConfigKey constant and GetCustomTypes() to storage interface
- Add IssueType.IsValidWithCustom() method for validation
- Add ValidateWithCustom() to Issue for combined status/type validation
- Update all validation call sites to use GetCustomTypes()
- Rename parseCustomStatuses to parseCommaSeparated for reuse

This enables Gas Town to register custom types like agent/role/rig/convoy
without hardcoding them in beads core, supporting the type extraction epic.

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/storage/sqlite/validators_test.go

@@ -205,14 +205,14 @@ func TestParseCustomStatuses(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got := parseCustomStatuses(tt.value)
+			got := parseCommaSeparated(tt.value)
 			if len(got) != len(tt.want) {
-				t.Errorf("parseCustomStatuses() = %v, want %v", got, tt.want)
+				t.Errorf("parseCommaSeparated() = %v, want %v", got, tt.want)
 				return
 			}
 			for i := range got {
 				if got[i] != tt.want[i] {
-					t.Errorf("parseCustomStatuses()[%d] = %v, want %v", i, got[i], tt.want[i])
+					t.Errorf("parseCommaSeparated()[%d] = %v, want %v", i, got[i], tt.want[i])
 				}
 			}
 		})