feat: add structured output support via action result schema

[jbguerraz]

Summary

Implements #128 - Structured Output via Action Metadata Schema

This PR adds support for declarative result schemas in action metadata, enabling structured JSON/YAML output and automatic text formatting.

Changes

• Add DefResult type and Result field to DefAction for parsing result schemas from YAML
• Add RuntimeResultProvider interface and WithResult composition for runtimes that return structured data
• Add NewFnRuntimeWithResult for function-based runtimes returning results
• Extend RunInfo to capture Result from RuntimeResultProvider
• Add --json and --yaml global flags via jsonoutput plugin for structured output
• Add outputJSON/outputYAML/outputText handlers in actionscobra plugin
• Generic text formatting: unwrap result wrappers and output key-value pairs
• Shell runtime: capture stdout as JSON when action has result schema
• Container runtime: capture stdout as JSON when action has result schema
• Add NewCapturingStreams helper for stdout capture
• Error output includes code field (e.g., EXIT_1) when available

Usage

Actions define a result schema in their YAML:

result:
  type: object
  properties:
    packages:
      type: array
      items:
        type: object
        properties:
          name:
            type: string
          ref:
            type: string

Go-based actions

Use NewFnRuntimeWithResult to return structured data:

action.SetRuntime(action.NewFnRuntimeWithResult(func(ctx context.Context, a *action.Action) (any, error) {
    // ... execute action
    return result, nil
}))

Shell-based actions

Simply echo valid JSON to stdout:

#!/bin/bash
echo '{"packages": [{"name": "foo", "ref": "v1.0.0"}]}'

Container-based actions

Same as shell - echo valid JSON to stdout from within the container:

CMD ["sh", "-c", "echo '{\"status\": \"ok\"}'"]

Output Formats

Output is handled automatically by launchr:

• Default: key-value text format
• --json: structured JSON wrapped in {"result": ...} or {"error": ...}
• --yaml: structured YAML with result: or error: keys

Error Handling

On error with --json:

{
  "error": {
    "message": "action failed with exit code 1",
    "code": "EXIT_1"
  }
}

Exit code is preserved for proper shell scripting.

Example

# Text output (default)
$ myctl model:list
name: plasma-core
ref: prepare
name: plasma-work
ref: 1.16.2

# JSON output
$ myctl model:list --json
{
  "result": {
    "packages": [
      {"name": "plasma-core", "ref": "prepare"},
      {"name": "plasma-work", "ref": "1.16.2"}
    ]
  }
}

# YAML output
$ myctl model:list --yaml
result:
  packages:
    - name: plasma-core
      ref: prepare
    - name: plasma-work
      ref: 1.16.2

Backward Compatibility

Actions without result schemas continue to work as before.

Coverage of Issue #128

Requirement	Status
Parse result field from action.yaml	✅
--json global flag	✅
--yaml global flag	✅
RuntimeResultProvider interface	✅
Shell runtime support	✅
Container runtime support	✅
Error handling with code	✅
Stream separation (stdout/stderr)	✅
Backward compatibility	✅

Closes #128

[jbguerraz]

and some love for the ci as benefits :)