feat: support previewConfig.target.path

[lintermansjens]

Summary

This PR adds support for storing preview environments under a configurable subpath (previewConfig.target.path) instead of only in the deployment repo root.

Why

Some deployment repos keep manifests in certain folders.
Before this change, preview commands only worked with root-level folders.

What changed

Added previewConfig.target.path parsing and validation in GitOps config.
Added path resolution logic to combine configured target path + generated preview namespace

Updated:

• create-preview to create/update files at the resolved path
• delete-preview to delete previews at the resolved path
• Improved logs to show preview namespace and resolved folder path.

Added tests for:

• default root behavior
• path parsing/validation (v1/v2)
• create/delete preview with configured target path

Updated docs:

• preview configuration now documents target.path
• getting-started command list reflects current CLI commands

Compatibility

Default behavior is unchanged: if previewConfig.target.path is not set, previews are still created in the repository root.