feat: custom providers, error handling, internal cleanup

[dkmnx]

Summary

Adds support for custom provider definitions, fixes silent error swallowing in config loading, and includes extensive internal refactoring and documentation improvements.

Changes

• feat(providers): support custom provider definitions in config.yaml
• fix(cmd): surface loadConfigOrEmpty errors instead of swallowing them
• refactor(providers): compute provider order from priority list
• refactor(cmd): remove dead code (setupSignalHandler, harnessBinary, yoloFlag rename), consolidate harness logic with crypto interface, add StartSession tests
• docs: fix markdown formatting, switch contributing guide to Google Go Style, document custom_providers
• test(execution): skip signal-self test on Windows

Test Plan

• go test -race ./... — all pass
• golangci-lint run ./... — 0 issues
• coverage 83.8% (above 70% threshold)
• Windows signal test correctly skipped
• Custom provider round-trip through config load/save

Notes

Bumps minor version (v2.7.0 → v2.8.0) due to the new custom providers feature. No breaking changes.