docs: fix Cursor init flag and document Windows paths

[ivanclarksoft]

Summary

Fixes documentation gaps from #3429 (docs items only):

1. roubleshooting.md:
   tk init --global --cursor does not exist — use --agent cursor (consistent with supported-agents docs).
2. configuration.md: add Windows config path %APPDATA%\rtk\config.toml.
3. gain.md: document Windows/macOS/Linux history.db locations.

Notes

• RuboCop autocorrect alias / struct field fix from RuboCop autocorrect aliases missed, rtk init --cursor in troubleshooting does not exist, Windows config path undocumented — plus what I could not verify and why #3429 left for a separate code PR.
• Docs-only.

Fixes #3429

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