feat(engine): update default model to claude-opus-4.7

[Copilot]

Summary

Bumps DEFAULT_COPILOT_MODEL from claude-opus-4.5 to claude-opus-4.7, used when an agent's front matter does not specify engine.model.

• Code: updated the constant in src/engine.rs and the two engine tests asserting the default --model flag value.
• Docs: refreshed default-model references in docs/{engine,front-matter,template-markers,local-development}.md.
• Prompts: refreshed default-model references in prompts/{create,debug,update}-ado-agentic-workflow.md.
• Doc-comment: updated the object-form example in src/compile/types.rs.

Test fixtures that pass an explicit model: claude-opus-4.5 were left alone — they exercise the override path, not the default.

Test plan

cargo build, cargo test, cargo clippy --all-targets --all-features all pass.