feat: add skill-customizer skill for domain-specific skill adaptation

[Fernando280773]

Summary

• Adds a new skill-customizer skill to the marketingskills plugin
• Automates customizing any marketing skill for a specific company from a website URL
• Eliminates repeated "what company do you work for?" setup on every skill invocation

What changed

A new skill at skills/skill-customizer/SKILL.md that:

1. Accepts a website URL + skill name(s) — fetches the site and extracts company name, product, ICP, pricing, brand voice, and competitors automatically
2. Writes a local override to ~/.claude/skills/<skill-name>/SKILL.md — this path takes precedence over the installed plugin version, so the customization is active immediately with no extra config
3. Covers 7 skill types with tailored customization patterns: competitive-intelligence, cold-email, cro, email-sequence, customer-research, pricing-strategy, content-strategy
4. Supports batch mode — customize multiple skills at once, with business-type presets (SaaS, e-commerce, services, marketplace)

Why

Users installing the marketingskills plugin currently re-enter their company context every time they invoke a skill. This skill solves that by turning a one-time URL input into a persistent, pre-filled override — the skill "knows" the company on all future invocations.

Reviewer notes

• The skill writes to ~/.claude/skills/ (user's local machine), not the repo — no repo files are modified at runtime
• Follows all naming rules: lowercase, hyphens, matches directory name
• SKILL.md is under 500 lines
• Description includes trigger phrases per spec

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