[codex] migrate CI to Nimby

[treeform]

What changed

• Migrated build CI to treeform/setup-nim-action@v6 with Nimby dependency installation from the workspace parent.
• Standardized CI to run nim r tests/tests.nim across Ubuntu, macOS, and Windows.
• Migrated docs CI to setup-nim v6, Nimby install, workflow_dispatch, and the required nim doc command.
• Renamed tests/test.nim to tests/tests.nim as the standard test entrypoint.

Validation

• cd ..; nimby install -g ws/ws.nimble
• nim r tests/tests.nim
• nim doc --index:on --project --git.url:https://github.com/treeform/ws --git.commit:master --out:.gh-pages src/ws.nim