docs: add pioneer charter dungeon design

[JOY (JOY)]

Summary

• add a Gate, Dungeon, and Pioneer Charter design doc for ranked Gates, first-clear rights, and in-game-only Clearance Royalty loops
• update the GDD, systems index, vertical-slice scope, roadmap, changelog, and GitBook navigation to include the new design surface
• record genre reference guidance from modern Tower/Gate/System manhwa while explicitly excluding real-world revenue-share semantics

Verification

• npx --yes markdownlint-cli2 README.md ROADMAP.md CHANGELOG.md CONTRIBUTING.md SECURITY.md AGENTS.md .claude/CLAUDE.md "docs/**/*.md"
• git diff --check
• codex review --uncommitted (clean after addressing initial review findings)