feat: add 'cat' command for displaying package resources

[aravindtga]

Restores the cat subcommand under pkg that was removed in #2393 (#2393), with small correctness and testability fixes on top.

Changes

• Restore thirdparty/cmdconfig/commands/cmdcat/cmdcat.go and wire it into pkg.
• Non-YAML/JSON file args (e.g. README.md, Kptfile) now return a clear error and non-zero exit instead of warning + silently succeeding. Directory form is unchanged; Kptfile stays excluded from the resource stream.

Test

• go build, go vet, and go test pass for thirdparty/cmdconfig/commands/cmdcat/...
• Manually verified the commands in the CLI

AI usage disclosure

• AI assistance (Kiro CLI) was used to draft portions of this change, including exploratory edits, test scaffolding, and this PR description.

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[Copilot]

Pull request overview

Restores the kpt pkg cat subcommand (previously removed in #2393) to print package resources, with additional correctness behavior around rejecting non-resource file arguments and improved test coverage for edge cases.

Changes:

• Reintroduce thirdparty/cmdconfig/commands/cmdcat implementation and hook it into kpt pkg.
• Add comprehensive unit tests covering directory traversal, nested packages, annotation behavior, formatting/comment stripping, and error cases.
• Change behavior for non-YAML/JSON file arguments to return an error and non-zero exit.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
thirdparty/cmdconfig/commands/cmdcat/cmdcat.go	Restores the cat command implementation and adds stricter file-argument validation and contextual error diagnostics.
thirdparty/cmdconfig/commands/cmdcat/cmdcat_test.go	Adds tests covering core output behavior and multiple historical issues/edge cases.
commands/pkg/pkgcmd.go	Wires the restored cat command into the pkg command group.

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[Copilot]

Pull request overview

Restores the kpt pkg cat subcommand (previously removed in #2393) to print KRM resources from a file or package directory, along with a focused test suite to validate output formatting, annotation behavior, recursion behavior, and error cases.

Changes:

• Reintroduce cmdcat Cobra command implementation and package traversal wiring.
• Add comprehensive unit tests covering directory/file inputs, annotation behavior, recursion, and failure modes.
• Wire pkg cat into the kpt pkg command group.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
thirdparty/cmdconfig/commands/cmdcat/cmdcat.go	Implements the restored pkg cat command, including flag handling, package traversal, and output formatting.
thirdparty/cmdconfig/commands/cmdcat/cmdcat_test.go	Adds test coverage for core behaviors and key regressions/edge cases.
commands/pkg/pkgcmd.go	Registers pkg cat under the existing kpt pkg command group.

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