STEP v0.2.1 — PLAN-only Bootstrap

A narrow correction to the pre-1.0 public trial baseline.

• Defines PLAN.md as the only required WorkNode artifact at bootstrap entry.
• Requires bootstrap to create logs/ and derive STATE.md, then stop.
• Clarifies that bootstrap creates no per-step log; the first execution run creates it from PLAN.
• Rejects bootstrap over orphaned step logs instead of inventing execution state.
• Adds a PLAN-only seed example and updates the copyable agent trial.

v0.2.0 remains an immutable record of the earlier wording.