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/README.md

@@ -62,9 +62,12 @@ Requires Python ≥ 3.10. No third-party packages — the server is pure stdlib.
 | `sleep_unschedule` | remove the nightly cron entry |
 
 Default backend is `mock` (no API spend); the `claude`, `codex`, and `copilot`
-backends use the corresponding authenticated CLI and budget. The seven tools
-call the same `python -m skillopt_sleep` actions as the other shared-engine
-integrations.
+backends use the corresponding authenticated CLI and budget. The `handoff`
+backend runs the cycle with no model subprocess or API key — the engine writes
+pending model calls to `.skillopt-sleep-handoff/PROMPTS.md` + `pending.json`
+(exit code 3) and resumes after answers are placed in `answers/<id>.md`; re-run
+`sleep_run` with the same arguments to resume. The seven tools call the same
+`python -m skillopt_sleep` actions as the other shared-engine integrations.
 
 ## Data boundary