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/mcp_server.py

@@ -62,8 +62,8 @@ _TOOL_SCHEMA = {
     "properties": {
         "project": {"type": "string",
                     "description": "Project dir to evolve (default: cwd)."},
-        "backend": {"type": "string", "enum": ["mock", "claude", "codex", "copilot"],
-                    "description": "mock = no API spend (default); claude/codex/copilot = real."},
+        "backend": {"type": "string", "enum": ["mock", "claude", "codex", "copilot", "handoff"],
+                    "description": "mock = no API spend (default); claude/codex/copilot = real; handoff = session answers prompts, no API subprocess."},
         "scope": {"type": "string", "enum": ["invoked", "all"],
                   "description": "Harvest scope (default: invoked project only)."},
         "source": {"type": "string", "enum": ["claude", "codex", "auto"],