fix(coding-agent): retarget OpenAI automatic defaults

[code-yeongyu]

Summary

Retarget Senpi's implicit OpenAI and Codex startup defaults from GPT-5.5 to GPT-5.6 Sol. Explicitly saved GPT-5.5 selections remain supported and continue to resolve through the settings path.

Changes

• Update the openai and openai-codex entries in defaultModelPerProvider to gpt-5.6-sol.
• Extend the existing initial-model provenance test to prove the automatic path selects Sol while an explicit GPT-5.5 setting remains selected.
• Record the core fork divergence in the nearest changes.md tracker.

QA & Evidence

• Failing-first proof: temporarily restored the prior GPT-5.5 defaults and ran npm exec vitest -- --run test/model-resolver.test.ts from packages/coding-agent; observed 2 intended assertion failures (gpt-5.5 received instead of gpt-5.6-sol).
• Focused GREEN: the same command passed all 55 tests after restoring the implementation.
• Resolver surface: an executable source-level resolver probe returned:
  • automatic: openai/gpt-5.6-sol, provenance provider-default
  • explicit: openai-codex/gpt-5.5, provenance settings
• Real CLI QA: cli-smoke.mjs --self-test passed 8/8 and rpc-drive.mjs --self-test passed 4/4 in isolated sandboxes; the real auth file was unchanged.
• Static validation: npm exec tsc -- --noEmit, browser smoke, and git diff --check passed.

Local artifacts are under local-ignore/qa-evidence/20260822-gpt55-provider-default/ and are intentionally not committed.

Risks & Residuals

• GPT-5.5 is not removed from provider catalogs or explicit selection paths.
• The change affects only implicit provider-default and restore-fallback selection that consumes the same map.
• No model networking or real provider credentials were used in tests.

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Summary by cubic

Retargets the OpenAI provider default from GPT-5.5 to GPT-5.6 Sol so implicit selection follows the current recommendation. Old behavior: provider-default selected GPT-5.5; new behavior: it selects GPT-5.6 Sol; explicit GPT-5.5 settings still resolve unchanged.

Review notes

• Update defaultModelPerProvider in packages/coding-agent/src/core/model-resolver.ts for openai and openai-codex to gpt-5.6-sol.
• Add packages/coding-agent/test/provider-default-model-selection.test.ts to assert automatic vs. explicit selection; refresh expectations in packages/coding-agent/test/model-resolver.test.ts.
• Update packages/coding-agent/test/suite/interactive-mode-scoped-settings.test.ts to reflect the post-auth default of openai/gpt-5.6-sol.
• Document the change in packages/coding-agent/src/core/changes.md and packages/coding-agent/CHANGELOG.md.

Rollout

• No migration required; saved GPT-5.5 selections continue to work.
• Impact is limited to implicit provider-default and restore-fallback paths.

^{Written for commit dda3105. Summary will update on new commits.}