feat : add consistent exit codes

[vikashkumar2020]

Changes

Fixes : #2855
Implemented consistent exit codes across tkn. Added a new pkg/exitcode package defining codes 0–5 (success, general error, not found, invalid input, timeout, unauthorized) with a typed error that propagates through the call stack. All Kubernetes API calls in pkg/actions now classify errors at the boundary via exitcode.FromAPIError(). The root command wires up exitcode.CodeFrom() to drive os.Exit, and adds structured {"error":"…","code":N} JSON output to stderr when --output json is set. A reference doc at docs/exit-codes.md covers the full scheme with examples.

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Release Notes

The consistent exit codes with typed errors, k8s API error classification, and structured JSON error output for --output json is added

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[divyansh42]

@vikashkumar2020 thanks for the PR!
Could you please add release notes and link it to the corresponding GitHub issue?

[vikashkumar2020]

@vikashkumar2020 thanks for the PR! Could you please add release notes and link it to the corresponding GitHub issue?

added into description