Define consistent exit codes for all tkn commands

[vdemeester]

Summary

AI agents rely on exit codes to determine success/failure without parsing output. Currently tkn exit codes are inconsistent and undocumented.

Proposal

Define and document a consistent exit code scheme:

Code	Meaning
0	Success
1	General error / command failure
2	Resource not found
3	Invalid input / validation error
4	Timeout
5	Unauthorized / forbidden

Acceptance Criteria

• Audit all commands for current exit code behavior
• Implement consistent exit codes across all commands
• Document exit codes in tkn --help or a dedicated doc
• Ensure errors on stderr include structured info when --output json is set (e.g. {"error": "not found", "code": 2})

/kind feature