DADBEAR: extract delta helper prompts

[adamlevineagent]

Summary

• Move the delta.rs LLM helper prompt bodies into prompt files under chains/prompts/delta
• Add chains/defaults/delta-helpers.yaml as the declarative registry for the delta helper prompts
• Keep delta.rs orchestration and audit-dispatch behavior unchanged while rendering the extracted templates

Verification

• cargo check --lib
• cargo test --lib edge_survives_endpoint_supersession -- --nocapture
• cargo test --lib faq_link_survives_related_node_supersession -- --nocapture
• cargo test --lib targeted_evidence_resolves_real_source_not_placeholder -- --nocapture
• cargo test --lib stale_check_decision_ -- --nocapture
• cargo test --lib test_set_file_content_hash_preserves_node_ids_order_independent -- --nocapture
• cargo test --lib not_stale_pass_naturally_terminates_without_execute -- --nocapture
• cargo test --lib skip_abstain_naturally_terminates_without_execute -- --nocapture
• ruby -e 'require "yaml"; YAML.load_file("chains/defaults/delta-helpers.yaml"); puts "yaml ok"'
• git diff --cached --check

Note: cargo fmt --check is still blocked by pre-existing repo-wide formatting drift outside this change; I did not apply broad formatting churn.