tasks: define RFC for workflows #887

Placeholder ticket for writing an RFC to define 'recipes'. This maps most closely to the concept of a "prompt library" or "custom slash commands" in various other products.

Related issue coder/coder#19258 can be considered a "Product Reference Document".

Potential reference points:

• Roo Code custom instructions https://docs.roocode.com/features/custom-instructions
• GitHub Copilot user instructions files https://code.visualstudio.com/docs/copilot/copilot-customization#_use-instructionsmd-files
• Continue.dev custom prompts https://docs.continue.dev/customize/deep-dives/prompts
• Claude Code custom slash commands / $ARGUMENTS https://docs.anthropic.com/en/docs/claude-code/slash-commands#custom-slash-commands