fix(agent): avoid fabricated fallback plans

[ZhiXiao-Lin]

Summary

• keep Auto execution direct when structured pre-analysis is unavailable
• replace numbered placeholder fallback tasks with one executable step containing the original request when planning is explicitly enabled
• remove the obsolete fallback-step prompt template

Validation

• cargo fmt --all -- --check
• cargo test -p a3s-code-core test_fallback_plan
• cargo test -p a3s-code-core auto_mode_does_not_fabricate_a_plan_when_pre_analysis_is_unavailable
• cargo clippy -p a3s-code-core --lib --tests -- -D warnings