fix(cli): ensure "cagent new" always generates a YAML stub when model call fails (fixes #106)

[UmarFarooq-MP]

This PR fixes #106
, where cagent new could fail silently (or with confusing provider errors) and leave the user without any generated YAML file.

✅ Changes

Added fallback logic in root.NewNewCmd to always write a stub YAML if model generation fails.

Added creator.WriteStubAgentFile helper that:

Creates a minimal valid YAML.

Selects sensible defaults (openai/gpt-4o-mini, anthropic/claude-sonnet-4-0, google/gemini-2.5-flash, or dmr/ai/qwen3:latest) based on available credentials.

Improved error messaging so users know both why generation failed and that a stub file was created.

Reproduction (before fix)

./cagent new myagent.yaml
# -> fails with connection refused / 404 errors
# -> no myagent.yaml created

Behavior (after fix)

./cagent new myagent.yaml
# -> error reported
# -> stub agent file created at myagent.yaml

Generated stub:

version: "1"

agents:
  root:
    model: dmr/ai/qwen3:latest
    description: "TODO: Add description"
    instruction: "TODO: Add instructions"

Why this matters

Users are no longer left “empty-handed” if the model call fails.
Greatly improves onboarding UX: cagent new always leaves a file users can edit.