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

@@ -1,7 +1,7 @@
-# /sleep — SkillOpt-Sleep for Codex
+# /skillopt-sleep — SkillOpt-Sleep for Codex
 #
-# Custom prompt: copy this file to ~/.codex/prompts/sleep.md and invoke with
-# `/sleep` in the Codex CLI. ($ARGUMENTS is the text after /sleep.)
+# Custom prompt: copy this file to ~/.codex/prompts/skillopt-sleep.md and invoke with
+# `/skillopt-sleep` in the Codex CLI. ($ARGUMENTS is the text after /skillopt-sleep.)
 
 Run the SkillOpt-Sleep offline self-evolution cycle. Action: $ARGUMENTS
 (empty → "status").