fix(init): respect CLAUDE_CONFIG_DIR for global paths

[TechWizard9999]

Description

rtk init -g ignored CLAUDE_CONFIG_DIR and always targeted ~/.claude. This PR makes global Claude path resolution env-aware so installs/patches land in the intended config directory.

Related Issue

Closes #633

Changes Made

• updated Claude config resolver to honor CLAUDE_CONFIG_DIR when set
• added explicit validation for empty CLAUDE_CONFIG_DIR
• updated manual patch instructions to print the resolved settings.json path instead of always showing ~/.claude/settings.json
• added unit tests for env override, home fallback, and empty override rejection
• added changelog entry under Unreleased bug fixes

Files Changed

• src/init.rs - env-aware path resolution and tests
• CHANGELOG.md - unreleased bug-fix note for rtk init -g does not respect CLAUDE_CONFIG_DIR #633

Testing

• cargo fmt --all --check
• rtk cargo clippy --all-targets (2 pre-existing warnings in untouched files)
• rtk cargo test
• rtk cargo test init::tests::test_resolve_claude_dir_

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated
• No breaking changes introduced

cc @pszymkowiak for review