fix: create stub files on init and rename apiVersion to v1alpha1

[wiliyam]

Summary

Fixes #20 and #21

Issue #20 — init stub files

agentspec init generated an agent.yaml referencing $file:prompts/system.md but never created the file, breaking the quick-start flow at the generate step.

Changes:

• After writing agent.yaml, init now creates prompts/system.md with a sensible default system prompt
• If --eval was selected, also creates eval/datasets/qa.jsonl stub
• Skips creation if the file already exists (safe to re-run)

Issue #21 — rename apiVersion

The manifest apiVersion: agentspec.io/v1 signalled a stable API but the schema is still evolving.

Changes:

• Renamed to agentspec.io/v1alpha1 across all source, test, example, and demo files (39 files)
• Added backward-compat migration v1ToV1Alpha1 so existing manifests using agentspec.io/v1 are silently upgraded rather than failing validation