fix: replace non-existent gpt-5.3-codex with gpt-5.2-codex in fallback chains

[yamyr]

Summary

• gpt-5.3-codex does not exist on the GitHub Copilot provider (only gpt-5.2-codex is available), causing model resolution failures for users who rely on GitHub Copilot as their provider.

Problem

The following agents/categories reference gpt-5.3-codex which isn't available:

• hephaestus agent — sole fallback model, so the agent never activates
• ultrabrain category — first fallback fails
• deep category — first fallback fails AND requiresModel: "gpt-5.3-codex" prevents the entire category from activating
• unspecified-low category — second fallback fails

Fix

Replace all 5 occurrences of gpt-5.3-codex with gpt-5.2-codex (the latest available codex model on GitHub Copilot) in model-requirements.ts and update corresponding test assertions.

Changes

• src/shared/model-requirements.ts — 5 replacements (hephaestus fallback, ultrabrain fallback, deep fallback + requiresModel, unspecified-low fallback)
• src/shared/model-requirements.test.ts — Updated test descriptions and assertions to match

Verification

• All 28 tests in model-requirements.test.ts pass
• bun run build succeeds

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Summary by cubic

Replaced references to gpt-5.3-codex with gpt-5.2-codex to restore model resolution on the GitHub Copilot provider. This fixes activation for affected agents and categories.

• Bug Fixes
  • Updated fallback chains and requiresModel for hephaestus, ultrabrain, deep, and unspecified-low to use gpt-5.2-codex.
  • Adjusted unit tests to reflect the new model.
  • All model-requirements tests pass; build succeeds.

^{Written for commit 22031af. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: The PR changes model versions from gpt-5.3-codex to gpt-5.2-codex, which could introduce regressions not guaranteed safe.}