refactor(to-plan,to-issues): prune no-ops in vertical-slice framing

Fold the redundant 'thin vertical slice cuts through all layers, NOT
horizontal' intro sentence into vertical-slice-rules bullet 1 (it was
duplicating the bullet), and cut to-plan's 'no parallelism / top-to-bottom'
restatement — 'sequential' is already anchored as the leading word. Keeps
the shared core identical across the two sibling skills.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skills/engineering/to-plan/SKILL.md

@@ -26,18 +26,18 @@ Look for opportunities to prefactor the code to make the implementation easier.
 
 ### 3. Draft vertical slices
 
-Break the work into **tracer bullet** phases. Each phase is a thin vertical slice that cuts through ALL integration layers end-to-end, NOT a horizontal slice of one layer.
+Break the work into **tracer bullet** phases.
 
 <vertical-slice-rules>
 
-- Each slice delivers a narrow but COMPLETE path through every layer (schema, API, UI, tests)
+- Each slice cuts a narrow but COMPLETE path through every layer (schema, API, UI, tests) — vertical, NOT a horizontal slice of one layer
 - A completed slice is demoable or verifiable on its own
 - Each slice is sized to fit in a single fresh context window
 - Any prefactoring should be done first
 
 </vertical-slice-rules>
 
-Order the phases **sequentially**, in dependency order — each phase builds on the ones before it. There is no parallelism: a plain top-to-bottom sequence.
+Order the phases in dependency order — each builds on the ones before it.
 
 ### 4. Quiz the user