Reduce system prompt token overhead from skill files

[GeneralJerel]

Priority: P1 | Source: PR #21 self-review (F2)

Problem

PR #21 adds ~9,150 tokens (75% increase) to the system prompt. load_all_skills() concatenates ALL .txt files unconditionally — mechanical diagram guidance is injected even for non-mechanical queries.

• Pre-PR skill text: ~48.9 KB (~12,200 tokens)
• Post-PR skill text: ~85.5 KB (~21,400 tokens)

Suggested Approaches

• Conditional skill loading — only inject mechanical-illustration-skill.txt and svg-shape-library.txt when the query involves physical/mechanical subjects
• Trim the shape library to the 6-8 most common shapes (as the issue proposal itself suggests)
• Establish a token budget target for total skill text