Dry-run and validation for mutating operations

[vdemeester]

Summary

Agents should be able to preview the effect of mutating operations before executing them. A --dry-run flag on all mutating commands enables agents to validate actions and present them to users for confirmation.

Proposal

• Add --dry-run flag to all mutating operations (create, delete, start)
• Perform local validation before Kubernetes API calls
• Return would-be changes in structured JSON format
• Enable agents to preview actions before execution

Acceptance Criteria

• --dry-run flag available on all mutating commands
• Dry-run performs local validation and reports errors
• Output shows what would happen in structured JSON (when --output json is used)
• No actual Kubernetes API mutations occur during dry-run

Context

Part of #2849 — making tkn agent-friendly.

Note: Kubernetes already supports server-side dry-run (--dry-run=server). This may leverage that mechanism where applicable, but should also cover tkn-specific validation (e.g., parameter completeness for tkn pipeline start).

/kind feature