v1.3.3

feat(models): add Claude Fable 5.1 (v1.3.3)

Promote claude:fable to Fable 5.1 (claude-fable-5-1) and shift the previous
version (5) down to claude:fable-legacy so it stays selectable, matching the
opus rollover. Pricing is unchanged for the tier.

- seed.ts: Fable 5.1 / Fable 5 (Legacy); the Claude block's sort orders move
  down one to open a slot for the legacy tier.
- schema.ts: idempotent migration for existing DBs — the flag is only rewritten
  while it still holds claude-fable-5, and the sort_order updates touch only
  rows still sitting at their seeded default, so custom picks and hand-reordered
  lists survive.
- model-discovery.ts: LEGACY_TIERS covers fable, so CLI and API discovery derive
  a fable-legacy tier as they already do for opus.
- CHANGELOG: 1.3.3 entry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>