Rename 'Three-Tier Approach for Application Code' to 'Three-Tier Approach'

Six pointers across SKILL.md, CLAUDE.md, basic-setup.md, and
setup-workflow.md cite the section as "Three-Tier Approach" (quoted
exact form). The heading had the longer "...for Application Code"
suffix. Rename the heading to match the pointers rather than update
all six callers. The Gemfile version-pin case is already under its own
section ("The Canonical Case: Gemfile Version Pin") so the
"for Application Code" qualifier was redundant.

Author: JuanVqz

dual-boot/references/code-patterns.md

@@ -29,7 +29,7 @@ This is genuinely two-sided — the `7.1.0` pin fails resolution on the 7.0 side
 
 ---
 
-## Three-Tier Approach for Application Code
+## Three-Tier Approach
 
 When a version gap affects application code (not the Gemfile), pick the **lowest tier that works**. Lower tiers are cheaper to read, cheaper to clean up, and do not leave dead branches behind. Climb tiers only when the lower option actually breaks.