Support common skill formats

[kination]

Overview

Let devin load reusable prompt templates ("skills") from disk, so users can define and invoke repeatable task patterns without retyping system-level instructions.

Motivation

Tools like Claude Code's superpower skills and LangChain prompt templates show that structured, composable prompts dramatically improve consistency. devin should support a local equivalent.

Proposed Work

• Skill format: a Markdown file with YAML frontmatter

  ---
  name: write-tests
  description: Generate unit tests for a given function
  ---
  Write comprehensive unit tests for the following code. Use the project's existing test style...

• Skill discovery: scan ~/.devin/skills/ and ./.devin/skills/ on startup
• /skill <name> chat command — prepends the skill's prompt to the next user message
• devin --skill <name> "query" flag for single-turn mode
• Tab-completion for skill names in the REPL (stretch goal)

Notes

• Skill prompts inject into the message, not the system prompt, to stay compatible with apfel's fixed --system flag
• Format should stay close to existing skill conventions (claude-code superpowers, etc.) to make migration easy