feat: add Muse Spark support

[Abbondanzo]

Adds Muse Spark 1.2 support through Meta’s public Model API. The CLI configures an OpenAI-compatible Responses provider, reads credentials from META_API_KEY, validates supported Muse model IDs during preflight, and runs Muse through the existing review pipeline. Also updates setup guidance, workflow templates, and tests.

[github-actions]

🤖 AI code review

Decision: Ready for human review

Overall PR risk: Medium. The PR modifies shared credential-resolution paths by adding a meta provider entry and a new auth-config branch that all entries pass through. Default Anthropic behavior is unaffected and meta support is opt-in since no existing configs reference it. Potential issues include configs misusing META_API_KEY for non-meta providers and altered credential forwarding for meta entries; rollback is direct.

🔗 Review setup:

• This PR changes code the reviewer prompts cite (src, src/tests, src/core/auth.ts, templates, templates/config.jsonc, and 1 more). Check that the guidance quoting it is still correct.

No findings.

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This review is advisory — it never blocks a merge and never auto-approves.