fix: accept music-cover-free model and differentiate network errors

[MeloMei]

Summary

Closes #182.

Three fixes for the mmx music cover command:

1. Model whitelist: Add music-cover-free to VALID_COVER_MODELS (models.ts) and VALID_MODELS (cover.ts) so free-tier users can use the cover feature.
2. Network error differentiation: In handler.ts, route network-level ETIMEDOUT errors to ExitCode.TIMEOUT instead of ExitCode.NETWORK, and append err.cause.code to generic network error messages for better diagnostics.
3. Help text: Update --model option description to document both music-cover (paid) and music-cover-free (free tier).

Changes

• src/commands/music/models.ts — added music-cover-free to valid cover model set
• src/commands/music/cover.ts — added music-cover-free to inline validation + updated help text
• src/errors/handler.ts — ETIMEDOUT routes to TIMEOUT exit code; include cause code in network error message

Tests

• Added: musicCoverModel accepts music-cover-free as default
• Added: accepts music-cover-free model in dry-run
• Added: network-level ETIMEDOUT error routes to TIMEOUT exit code
• Added: generic network error includes cause code when available

All 362 existing tests still pass (2 pre-existing failures unrelated to this change).

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