Add the Fable model tier

[jellologic]

Problem

We set three model tiers. Claude Code has four — ANTHROPIC_DEFAULT_FABLE_MODEL exists and we never set it, so the Fable alias falls through to a provider default that may not exist on the backend.

Proposal

Add Fable as a fourth tier throughout: provider defaults, the wizard, the model picker, and buildEnv.

Acceptance criteria

• ANTHROPIC_DEFAULT_FABLE_MODEL set when configured
• Fable appears in the wizard and picker alongside the existing three
• Existing 3-tier configs load without migration and leave Fable unset
• Audit the full ANTHROPIC_DEFAULT_* family for anything else we've missed

Notes

Confirm whether CLAUDE_CODE_SUBAGENT_MODEL should follow Fable rather than Opus — subagents currently inherit our subagentFollowsOpus behaviour, which is a workaround for #34821.

[jellologic]

Done — landed in 382e58d. Fable is a first-class tier everywhere (TIERS = [opus, sonnet, haiku, fable]); ANTHROPIC_DEFAULT_FABLE_MODEL is set from the same table and the exhaustive TierRecord makes a missing tier a compile error. 3-tier configs migrate with Fable left unset; wizard/picker include it. CLAUDE_CODE_SUBAGENT_MODEL follows opus via subagentFollowsOpus (kept as the #34821 workaround).