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/devin-rules.snippet.md

@@ -0,0 +1,18 @@
+# SkillOpt-Sleep (Devin)
+
+You have access to a nightly self-evolution cycle via the `skillopt-sleep` MCP
+server. Use these tools to improve your long-term skills over time:
+
+- **`sleep_status`** — how many nights have run + the latest staged proposal
+- **`sleep_dry_run`** — preview a cycle without changing anything
+- **`sleep_run`** — run a full cycle; stages a proposal for review
+- **`sleep_adopt`** — apply the staged proposal to `.devin/skills/skillopt-sleep-learned/SKILL.md`
+- **`sleep_harvest`** — debug: list the recurring tasks mined from recent sessions
+
+When a user asks about the sleep cycle, skill evolution, or improving your
+long-term memory, prefer calling these tools over explaining the concept.
+
+Default backend is `mock` (no API spend). Pass `backend: "claude"` or
+`backend: "codex"` with your own API key for real LLM-driven optimization.
+
+Place this file at `.devin/rules/skillopt-sleep.md` in your workspace.