docs: add CHANGELOG.md with 3.12.0 release notes

[Copilot]

No structured release notes existed for 3.12.0. Adds CHANGELOG.md documenting the full release.

Contents

• New features — BlueMap/Dynmap web-map integration, SPAWN_PROTECTION and WIND_CHARGE flags, expel fallback command, blueprint paste commands, blueprint GUI pagination, admin max-team-size command, clickable home list, force-field fling, bypass-lock notification, geo-limit projectiles, paginated help, /island lock command
• Bug fixes — obsidian/lava duplication exploit (with new obsidianScoopingCooldown / obsidianScoopingRadius config keys), player XP not resetting on team join, sugar cane / cocoa beans / nether wart wrong protection flags, purge-unowned NPE, end/nether explosion NPE, island settings comparator crash, multi-line color-code stripping
• Configuration changes table — documents all three new config.yml keys with defaults and descriptions
• What's Changed — all ~40 merged PRs linked with authors, plus full-changelog comparison URL
• Formatted to match the 3.11.2 release style (compatibility, upgrading, legend sections)

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