Add Devin plugin (plugins/devin): MCP server + ATIF-v1.7 harvest

Wires the skillopt_sleep engine into Devin (Cognition) via an MCP server,
following the same thin-shell pattern as plugins/copilot.

- mcp_server.py: stdlib-only stdio MCP server exposing the standard sleep_*
  tools (status, dry-run, run, adopt, harvest). REPO_ROOT defaults to ../.. so
  it finds skillopt_sleep automatically when run from plugins/devin/.
- harvest_devin.py: converts Devin ATIF-v1.7 transcripts, agentmemory, and
  .devin/skills/*/SKILL.md into the Claude Code-compatible JSONL the engine
  consumes; enriches with taskKey + outcome envelopes (hard test/build signal
  or judge rubric). Workspace auto-detection; cross-platform paths.
- judge.py, mcp-config.example.json, devin-rules.snippet.md, README.md.
- plugins/README.md: add Devin to the platform + install tables.

No changes to skillopt_sleep; shells out to `python -m skillopt_sleep` like the
other plugins. Pure stdlib; default backend mock (no API spend).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/devin/mcp-config.example.json

@@ -0,0 +1,11 @@
+{
+  "mcpServers": {
+    "skillopt-sleep": {
+      "command": "python3",
+      "args": ["/abs/path/to/SkillOpt/plugins/devin/mcp_server.py"],
+      "env": {
+        "SKILLOPT_DEVIN_CLAUDE_HOME": "~/.skillopt-sleep-devin"
+      }
+    }
+  }
+}