feat: add Fable using host-owned model routing

[mychaelconnolly]

Sources and design precedent

• OpenAI codex-plugin-cc stable model routing forwards model names to the host, with only narrow convenience aliasing.
• Anthropic Claude Code model configuration documents aliases as provider-aware references that update over time.
• Anthropic Claude Fable 5 documents the model's availability and long-horizon coding use.
• Anthropic Claude Code commands defines /model as the interactive model and effort picker.

OpenAI and Anthropic alignment

This PR adds Fable using the host-owned model-routing boundary shared by OpenAI's official reciprocal plugin and Anthropic's Claude Code guidance.

The companion forwards fable directly to Claude Code instead of pinning claude-fable-5. Claude Code therefore owns the moving alias while the plugin retains its existing effort policy.

What changed

• Added fable to companion help, README guidance, and runtime skill contracts.
• Added high as the default Fable effort while preserving explicit overrides.
• Added unit and integration coverage for task, standard-review, and adversarial-review flows.
• Kept existing Opus, Sonnet, and Haiku routing unchanged.

User impact

Users can select Fable with --model fable across review and rescue workflows. No version-specific Fable model ID is embedded in the plugin.

Validation

• npm run check — 509 unit tests, 42 integration tests, and 22 E2E tests passed.
• Live no-tools smoke: --model fable --effort high completed successfully and resolved through Claude Code.

[sf-jin-ku]

Additive and host-owned: fable is forwarded to Claude Code rather than pinned to a version-specific ID, which is the same boundary the plugin now uses for the Codex child model. Merging ahead of #77; this ships in 1.4.0.