[#187] Fix export hint path and ExportError conformance

[bguidolim]

Context

Fixes #187. The mcs export command had two issues:

1. The post-export hint (mcs pack add <path>) printed the raw outputDir argument instead of the resolved absolute path, producing broken suggestions for relative paths.
2. ExportError used CustomStringConvertible instead of LocalizedError, so ArgumentParser could display generic error messages instead of the custom descriptions.

Changes

• Pass resolved outputURL.path to printPostExportHints() and use it in the hint
• Change ExportError to conform to LocalizedError with errorDescription, matching MCSError and ManifestError conventions

Testing Steps

• swift build passes
• Run mcs export ./relative-path and verify the hint shows an absolute path
• Trigger an ExportError (e.g. export to an existing directory) and verify the custom message displays

[Copilot]

Pull request overview

Fixes two user-facing issues in mcs export: the post-export “next steps” hint now prints the resolved output path, and ExportError now conforms to LocalizedError so ArgumentParser can display the intended custom error messages.

Changes:

• Pass outputURL.path into printPostExportHints() and use it in the mcs pack add ... hint.
• Update ExportError to conform to LocalizedError with errorDescription (aligning with other error types in the repo).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/mcs/Export/PackWriter.swift	Switch ExportError from CustomStringConvertible to LocalizedError for proper CLI error rendering.
Sources/mcs/Commands/ExportCommand.swift	Use the resolved output path in post-export hints so suggested commands work for relative paths.

Comments suppressed due to low confidence (1)

Sources/mcs/Commands/ExportCommand.swift:412

• The post-export hint interpolates outputPath directly into a shell command. If the path contains spaces (common on macOS), the suggested mcs pack add ... command will break. Consider quoting/escaping the path in the hint output (e.g., wrap in quotes when it contains whitespace, or use a small shell-escape helper for arguments).

        output.plain("  1. Review the generated techpack.yaml")
        output.plain("  2. Test with: mcs pack add \(outputPath)")
        output.plain("  3. Share via git: push to a repository and use mcs pack add <url>")

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