Deduplicate overlapping guidance between mechanical-illustration and svg-diagram skills

[jerelvelarde]

Priority: P2 | Source: PR #21 self-review (F4)

Problem

mechanical-illustration-skill.txt and svg-diagram-skill.txt have significant conceptual overlap. Both now cover:

• Shape construction rules
• 3-tier annotation hierarchy
• Sub-diagram standards
• Force arrow conventions
• Composition zones

The mechanical skill repeats guidance already present in the expanded svg-diagram-skill, with fuller examples.

Suggested Fix

Keep the architectural rules in svg-diagram-skill.txt only. Reduce mechanical-illustration-skill.txt to reference those rules and add only domain-specific worked examples (car, airplane compositions).