fix(config): update embedded Anthropic model

[owainlewis]

Summary

• update the embedded Anthropic model from claude-opus-4-8 to claude-opus-5
• keep the developer configuration example aligned with the embedded default
• make the embedded fallback regression test assert the declared default model exactly

Why

A clean installation used the stale model pinned in the embedded YAML instead of Neo's current Anthropic default. Explicit models in project or user configuration remain unchanged.

Test plan

• go test ./internal/config ./cmd/neo
• go test ./...
• git diff --check
• go build -o /tmp/neo-issue-229.../neo ./cmd/neo
• run the built binary from an empty temporary working directory with an empty HOME; neo doctor reports loaded embedded and pass model claude-opus-5

Risks

Low. This changes only the embedded default used when no project or user configuration exists. Explicit configured models are preserved.

Related issue

Closes #229

[owainlewis]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: f52d6dcb06

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