evals: load eval prompts from disk

[wgqqqqq]

Summary

• add runtime prompt loading for CLI/eval prompts via --prompts-dir and BITFUN_PROMPTS_DIR
• include runtime prompt file fingerprints in prompt cache identity
• add full model exchange tracing override for request-body validation

Verification

• cargo test -p bitfun-core runtime_prompt_override_reaches_anthropic_request_body_system_field -- --nocapture
• cargo check -p bitfun-cli
• cargo test -p bitfun-cli prompts -- --nocapture
• git diff --check

Manual validation

• Ran bitfun-cli with /tmp/bitfun-prompt-test/agentic_mode.md and verified the Anthropic request body top-level system field was overridden in the request trace.