refactor(plugins): remove per-package registry singletons and legacy function variables

[indaco]

Description

Remove legacy global state from the plugin system and command infrastructure. This eliminates per-package registry.go singleton files (dead code superseded by the centralized PluginRegistry) and migrates mutable var XxxFn function variables to plain exported functions, improving testability and removing race condition risks.

Related Issue

• None

Notes for Reviewers

• Per-package registries deleted: tagmanager, commitparser, versionvalidator, releasegate, dependencycheck, auditlog, changeloggenerator, changelogparser - none were called from production code
• FromConfigStruct() moved from auditlog/registry.go to auditlog/config.go (only production-used symbol from deleted files)
• Function variable migrations:
  • RunPreReleaseHooksFn -> RunPreReleaseHooks
  • FromCommandFn -> FromCommand
  • LoadConfigFn -> LoadConfig
  • SaveConfigFn -> SaveConfig
  • LoadPreReleaseHooksFromConfigFn -> LoadPreReleaseHooksFromConfig
• Test updates: bump and plugin tests now use PluginRegistry directly instead of overriding per-package GetXxxFn globals