[codex] migrate ci to nimby

[treeform]

Summary

• Replace the legacy CI setup with actions/checkout@v5, treeform/setup-nim-action@v6, and Nimby dependency installation from the GitHub workspace parent.
• Add ubuntu-latest, macos-latest, and windows-latest coverage for the standard build workflow.
• Add the docs workflow using the required Nim doc command and gh-pages publishing.
• Rename tests/test.nim to tests/tests.nim so CI can run the standard test entrypoint.

Validation

• nimby install -g "steganography/steganography.nimble" from C:\p
• nim r tests/tests.nim
• nim doc --index:on --project --git.url:https://github.com/treeform/steganography --git.commit:master --out:.gh-pages src/steganography.nim
• git diff --check
• git diff --cached --check