feat(providers): discover VSCodium storage paths

[ozymandiashh]

Summary

VSCodium uses the same VS Code-compatible storage layout, but under VSCodium/User/... instead of Code/User/.... CodeBurn was only scanning VS Code roots for these providers, so VSCodium users could have valid Copilot, Roo Code, or KiloCode sessions on disk that never showed up in reports.

This adds the missing VSCodium roots for Copilot workspaceStorage transcripts and Cline-family Roo/Kilo globalStorage task history. Roo/Kilo also get VS Code Insiders globalStorage parity with Copilot. Missing roots are still skipped quietly, so users with only one editor installed do not pay a behavior penalty.

The path helpers now accept injected homeDir and platform values so the macOS, Windows, and Linux roots can be tested deterministically. The Cline-family discovery test also exercises scanning multiple storage roots in one run.

Closes #230.

Validation

• npx tsc --noEmit
• npx vitest run tests/providers/vscode-cline-parser.test.ts tests/providers/copilot.test.ts tests/providers/roo-code.test.ts tests/providers/kilo-code.test.ts
• npx vitest run
• npm run build