refactor(cc): simplify CC generators by extracting shared helpers

Extract common patterns duplicated across CC generators into shared functions:
- ReadyExtensions() on ExtensionCache replaces inline filtering and readyExts()
- GetVersion() / GetPkgURLLink() on Extension replace repeated null-check patterns
- InferRepo() consolidates duplicate inferRepo methods from cc_page.go and cc_os.go
- WriteMarkdownFile() replaces repeated os.MkdirAll + os.WriteFile boilerplate
- runWithCache() in cmd/cc.go extracts InitDB+LoadCache boilerplate from 6 commands
- CCExtensionTable uses shared helpers, removing duplication in cc_markdown.go
- Remove NullBool/NullString type aliases in favor of direct sql.Null* types

Net reduction: ~350 lines removed across 9 files with no functional changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Vonng

cli/cache.go

@@ -246,6 +246,17 @@ func buildDependencyMap(cache *ExtensionCache) {
 	}
 }
 
+// ReadyExtensions returns all extensions that are not in "not-ready" state
+func (cache *ExtensionCache) ReadyExtensions() []*Extension {
+	var exts []*Extension
+	for _, ext := range cache.Extensions {
+		if ext.IsReady() {
+			exts = append(exts, ext)
+		}
+	}
+	return exts
+}
+
 // GetSiblingExtensions returns other extensions that share the same package
 func (cache *ExtensionCache) GetSiblingExtensions(pkgName, currentExtName string) []*Extension {
 	var siblings []*Extension