refactor(fs): replace process-wide os.Chdir with explicit base path

[ManthanNimodiya]

Description

This PR addresses #1269 by eliminating process-wide os.Chdir calls in pack, unpack, and kitimport operations in favor of explicit base paths.

Changing the process working directory via os.Chdir mutated the caller's working directory and prevented safe concurrent execution.

Key Changes

• Pack pipeline: Added ContextDir to SaveModelOptions in local-storage.go; updated tar and raw layer writers to resolve and walk paths relative
  to contextDir. Removed os.Chdir from pkg/cmd/pack/cmd.go and pkg/cmd/kitimport/util.go.
• Unpack pipeline: Removed os.Chdir from UnpackModelKit and unpackRemote in core.go. Layers are now explicitly extracted and verified into
  target unpack destinations.
• Tests: Added TestPackUnpack_PreservesWorkingDir in pack-unpack_test.go verifying that neither kit pack nor kit unpack alters the process
  working directory.

Testing

• go test ./pkg/... passed
• go test ./... passed (including all integration tests in testing/)

Fixes #1269

[ManthanNimodiya]

@amisevsk, @bmicklea have a look when you get time