feat(agent): update model catalog -- add claude-sonnet-4.6, gpt-5.4, gpt-5.3-codex

[tamirdresher]

Summary

Updates the model catalog in squad.agent.md to include recently-added Copilot models.

Changes

Valid models list

• Added claude-sonnet-4.6 (new standard default)
• Added gpt-5.4 and gpt-5.3-codex to Standard tier

Fallback chains

• Standard: claude-sonnet-4.6 -> claude-sonnet-4.5 -> gpt-5.4 -> gpt-5.3-codex -> claude-sonnet-4 -> gpt-5.2 -> (nuclear)

Platform default note

• Updated to reflect claude-sonnet-4.6 as current platform default

Background

The models.ts catalog in squad-sdk already includes these models. This PR aligns the agent instructions with what is available in the actual model registry.

Fixes #588

[tamirdresher]

Closing — bradygaster has opened #619 which addresses the same model catalog update (#588). Deferring to the upstream-maintained PR.