feat(devin): expose handoff backend in MCP server enum (#3)

The Devin MCP server's backend enum only listed mock/claude/codex/copilot,
excluding the handoff backend that was merged to the engine in #125. This
made the subscription-friendly, no-API-key path unavailable to Devin users
while Claude Code had a dedicated /skillopt-sleep-handoff command for it.

Add "handoff" to the backend enum in _TOOL_SCHEMA so sleep_run accepts
backend: "handoff". The engine already handles the prompt/answer file loop
(exit code 3 + .skillopt-sleep-handoff/); the MCP server needs no special
handling — it passes through the engine output showing pending prompts.

Update the README, rules snippet, and test_backends_in_enum accordingly.

plugins/devin/devin-rules.snippet.md

@@ -24,7 +24,10 @@ Always pass the absolute Devin workspace as `project`, especially for
 `sleep_adopt`. Default backend is `mock` (no provider calls). The `claude`,
 `codex`, and `copilot` backend values use the corresponding installed and
 authenticated CLI; they do not require this plugin to implement a separate
-API-key flow.
+API-key flow. The `handoff` backend runs the cycle with no model subprocess
+or API key — the engine writes pending model calls to
+`.skillopt-sleep-handoff/` and exits; answer each prompt in a fresh context
+and re-run `sleep_run` to resume (typically 3–6 rounds).
 
 The Devin conversion and mock workflow stay local. A real backend sends
 truncated transcript excerpts and derived tasks to the selected provider for