refactor: eliminate shared mutable state via struct-based DI

[indaco]

Description

Replace package-level function variables with struct-based dependency injection across packages. This eliminates shared mutable state and made dependency flow implicit.

Changes by package:

• changeloggenerator: new GitOps struct encapsulates git function fields; IsInteractiveFn/ConfirmMergeFn moved to plugin struct fields
• dependencycheck: format-specific read/write function vars moved to DependencyCheckerPlugin struct fields, initialized in constructor
• commitparser/gitlog: new GitLog struct with ExecCommandFn field; GetCommitsFn type and DefaultGetCommitsFn() factory for callers
• tagmanager: nowFunc global replaced with optional NowFunc parameter on NewTemplateData()
• commands/bump: new bumpDeps struct with context-based injection via context.WithValue; holds inference, bumper, and commit retrieval functions
• extensions: LoadExtensionManifestFn global var replaced with exported LoadExtensionManifest() function; all callers updated
• extensionmgr: removed copyDirFn and defaultFileCopier global vars; tests use NewOSFileCopier() or MockFileCopier directly
• cli/config tests: replaced manual os.Getwd/os.Chdir/os.Unsetenv with t.Chdir() and t.Setenv() for proper test isolation

Related Issue

• None

Notes for Reviewers

• Only readFileFn/writeFileFn in dependencycheck/parsers.go remain as package-level vars (adapter-level, not plugin-level)