feat(model-overlays): add gpt-5.6 overlay + resolver mapping

[chrisquorum]

What

OpenAI's current flagship is GPT-5.6 (tiers: Sol / Terra / Luna), generally
available across ChatGPT, Codex, and the API since ~2026-07-09. Today gpt-5.6*
resolves through models.ts to the generic gpt overlay, missing the GPT-5.x
family's anti-verbosity tuning. This adds a dedicated gpt-5.6 overlay so it
inherits the gpt base plus the terse-output layer — the same treatment
gpt-5.4 already gets.

Changes

• model-overlays/gpt-5.6.md (new) — {{INHERIT:gpt}} + the anti-verbosity /
  shortest-form-answer nudges (mirrors gpt-5.4.md).
• scripts/models.ts — add gpt-5.6 to ALL_MODEL_NAMES and a resolveModel
  mapping (/^gpt-5\.6(-|$)/ → gpt-5.6, matching the Sol/Terra/Luna tier
  suffixes), placed before the generic gpt fallback; doc-comment updated.

Notes

• Content mirrors gpt-5.4's family nudges — a family-inheritance overlay, not
  empirically re-tuned for 5.6. Retune as 5.6-specific patterns emerge.
• Follows the gpt-5.4 precedent: no dedicated overlay test (the gpt-*
  overlays don't carry one; the opus-* overlays do). Resolver + generated-doc
  behavior is validated below.

Validation

• resolveModel('gpt-5.6'), 'gpt-5.6-sol', 'gpt-5.6-terra' → gpt-5.6;
  gpt-5.4, gpt-4o, and bare gpt unchanged.
• Resolved gpt-5.6 overlay inherits the gpt base (Completion bias) and
  carries the Anti-verbosity protocol, with no unresolved {{INHERIT:...}}.
• bun run gen:skill-docs → no generated-doc churn (only the two files above);
  the docs-freshness CI gate stays green.
• bun run typecheck → no new errors in the changed files.

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[time-attack]

honestly not needed. just because in my testing i havent seen a marginal difference in how opus 4.8 behaves. might be needed for fable
@16francej close