Simplify workflows to code step runner

[attmous]

Summary

• replace the old multi-actor workflow surface with the single code workflow and coder actor
• remove stale orchestrator, routing, action, board, and legacy workflow template modules
• update schema, bootstrap, prompt/runtime/status surfaces, skills, plugin help, and docs for the step model

Validation

• schema/config load for packages/core/src/sprints/workflows/templates/code.md
• uv run ruff check packages/core/src/sprints/core packages/core/src/sprints/workflows packages/core/src/sprints/skills packages/core/src/sprints/observe packages/core/src/sprints/services packages/cli packages/plugins/hermes
• uv run python -m compileall packages/core/src/sprints/core packages/core/src/sprints/workflows packages/core/src/sprints/observe packages/core/src/sprints/services packages/cli/src packages/plugins/hermes/src
• git diff --check