feat(sleep): SkillOpt-Sleep plugin update (preview) — engine robustness + scheduling

Updates the SkillOpt-Sleep plugin on top of the current main. User-facing and
engine improvements since the initial drop:

* Command renamed /sleep -> /skillopt-sleep across Claude Code + Codex shells;
  refreshed plugin READMEs and install scripts.
* Built-in scheduling (skillopt_sleep/scheduler.py + __main__): schedule /
  unschedule the nightly cycle without external cron wiring.
* Backend robustness: bounded retry with backoff (no more silent empty-string
  on transient 429/timeout), content-filter-safe rollout prompt, an
  output-contract guardrail that rejects edits violating the task's required
  format, and a per-sample cache key so repeated dream rollouts are independent
  samples (fixes degenerate single-sample reflection).
* consolidate / rollout / replay: parallel multi-rollout dreaming, gate-mode
  controls, TaskRecord.system framing field.

Scope: this commit ships only the plugin engine + shells. Research/benchmark
harnesses and their data are intentionally not included; the public package
has no dependency on them (the one research-evaluator import is now guarded).
Marked as an early preview in the README; we'll keep iterating.

99/99 unit tests pass.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

plugins/codex/prompts/skillopt-sleep.md

@@ -0,0 +1,21 @@
+# /skillopt-sleep — SkillOpt-Sleep for Codex
+#
+# Custom prompt: copy this file to ~/.codex/prompts/skillopt-sleep.md and invoke with
+# `/skillopt-sleep` in the Codex CLI. ($ARGUMENTS is the text after /skillopt-sleep.)
+
+Run the SkillOpt-Sleep offline self-evolution cycle. Action: $ARGUMENTS
+(empty → "status").
+
+Use the bundled runner via shell:
+
+    bash "${SKILLOPT_SLEEP_REPO:?set SKILLOPT_SLEEP_REPO to the repo root}/plugins/run-sleep.sh" $ARGUMENTS --project "$(pwd)"
+
+Then:
+- For `run`/`dry-run`: read the staged `report.md` and show the held-out
+  baseline → candidate score and the proposed edits. `run` only stages a
+  proposal; nothing live changes until `adopt`.
+- For `adopt`: confirm which files were updated and that a backup was written.
+- Never edit the user's AGENTS.md / skills yourself; only `adopt` does that.
+
+Default backend is `mock` (no API spend). Add `--backend codex` for real
+improvement on the user's Codex budget.