refactor: simplify create-expert pipeline — remove plan, flatten structure

Major simplification of the create-expert pipeline:

- Remove plan expert — write handles both planning and authoring
- Write produces perstack.toml + test-spec.md (test query + signals)
- Write delegates to review directly (soft signal as direct delegation)
- Simplify delegate names: write, review, test, verify
- Remove domain knowledge design principles, focus on signal design
- Hard signal redesign:
  - Must: ONE signal, binary, core task completion. Always blocks.
  - Should: multiple signals, user value validation, threshold-based sign-off
- Soft signal: recommended pattern is executor delegates to reviewer directly

Before: create-expert → {plan, write-definition, review-definition, test-expert, verify-test}
After:  create-expert → {write → review, test, verify}

Version 1.0.22.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: FL4TLiN3