Add schema introspection command (tkn schema)

[vdemeester]

Summary

AI agents need to discover API schemas at runtime without consuming documentation tokens. A tkn schema command would make the CLI self-documenting.

Proposal

# Show PipelineRun CRD schema
tkn schema pipelinerun

# Show what tkn pipeline start accepts
tkn schema pipeline.start

# List all available resources
tkn schema --list

Output should be machine-readable JSON containing field names, types, required/optional status, and descriptions.

Implementation Ideas

• Use Tekton API server discovery (/openapi/v2 or /openapi/v3)
• Parse CRD schemas from the cluster
• Optionally bundle a fallback static schema for offline use

Value

• Agents can self-serve API structure without pre-stuffed documentation
• Reduces hallucinated field names and values
• Schema is always in sync with the actual cluster version

/kind feature

[vdemeester]

Closing for now — kubectl explain pipelinerun.spec already covers this use case for Tekton's small set of CRDs. Can revisit if there's demand.