docs(marketing): add AppNiche ASO data source

[skiffer]

Summary

• Add AppNiche as an optional ASO data source for iOS app intelligence, revenue/download estimates, keyword metrics, rankings, reviews, and competitor signals.
• Link the ASO skill to AppNiche API/MCP docs where external ASO tools are already referenced.
• Add a missing trigger clause and Anti-Patterns section so the skill clears the repository's quality target.

Context

Disclosure: I work on AppNiche/GetAppNiche. This addition is suggested because the App Store Optimization skill already references external ASO tools such as AppTweak, Sensor Tower, and data.ai for ranking/volume/competitor data. AppNiche fits as an iOS-focused API/MCP data source and remains optional.

Changes

• Updated marketing/app-store-optimization/SKILL.md data constraints and integration summary.
• Updated marketing/app-store-optimization/references/operations-and-benchmarks.md integration matrix.
• Added a concise trigger clause and Anti-Patterns section.

Testing

• Local quality checks passed
• No secrets, credentials, or API keys committed
• Skill SKILL.md files updated

Testing Details:

git diff --check
python scripts/skill_quality_scorer.py marketing/app-store-optimization
# Score: 73/100, up from pre-existing 65/100

Security

• No secrets, credentials, or API keys committed
• No destructive commands in generated outputs
• Dependencies reviewed (none added)

Documentation

• Skill SKILL.md files updated

Type: docs
Scope: marketing/app-store-optimization