Feat/zt 90 plugin renderers

[zaynelt]

Add support for optional plugin-managed renderers

Summary:

• Adds optional command/subcommand renderer support
• Defines and validates renderer RPC contract
• Adds reusable render, warning and deterministic JSON fallback behaviors
• Wires changes through dynamic plugin commands, recipes validate, activation diagnostics
• Adds contract and behavior tests
• Updates example plugin, README, ADR-004, CHANGELOG

Closes #90

Checklist

• go build ./... and go test ./... pass
• Docs / CHANGELOG.md updated as needed
• ADR check: if this changes behavior documented in an ADR (docs/adrs/ADR-*.md), I amended that
  ADR in place — a dated amendment that preserves the original text — in this PR, and recorded
  myself on Amended-by (not Author(s)).
  See docs/adrs/ADR-000-how-we-use-adrs.md. (N/A if no ADR
  covers this change.)