Default Codex to GPT-5.5

[isthatyousaf]

Change description

• Adds GPT-5.5 as the first built-in Codex model, default Codex model, and 5.5 mention/model alias.
• Shares Codex Fast mode capabilities across GPT-5.x Codex models so GPT-5.5 supports Fast mode.
• Keeps Codex built-in selector ordering ahead of runtime discovery ordering, so GPT-5.5 stays first even when model/list returns another order.

Test results

• bun run test from the repo root passed.
• bun fmt, bun lint, and bun typecheck were not run because the repo instructions say not to run them unless explicitly requested.

Deployment impact

• No deployment or migration steps expected. Existing legacy fallback normalization now uses GPT-5.5 for ancient Codex thread-created events without model data.

Review focus

• Codex default-model fallback paths in server/web persistence.
• Dynamic model-list merge behavior in apps/web/src/providerModelOptions.ts.
• Whether the selector ordering should remain static-first for all future Codex runtime models.