Add consistent --output json support to all tkn commands

[vdemeester]

Summary

Not all tkn commands support --output json. This makes it unreliable for AI agents and scripts that need machine-readable output.

Current State

• start --dry-run supports --output json/yaml
• list commands use Go template tables, some support --output via PrintObjects
• describe commands have custom formatted output, some support --output via PrintObjectV1
• logs, delete, cancel have no structured output

Proposal

1. Add --output json and --output yaml to every command that produces output
2. Auto-detect non-TTY stdout and default to JSON (opt-out with --output table)
3. Support TKN_OUTPUT_FORMAT=json environment variable for session-wide default
4. Ensure all JSON output goes to stdout, all status/progress messages to stderr

Commands to update

• pipeline list/describe/start/delete
• pipelinerun list/describe/delete/cancel/logs
• task list/describe/start/delete
• taskrun list/describe/delete/cancel/logs
• customrun list/describe/delete
• bundle list/push
• eventlistener list/describe
• triggerbinding list/describe
• triggertemplate list/describe
• clustertriggerbinding list/describe

/kind feature