Move to nxplain-sh and add release and contribution tooling

[miguelmartens]

Updates the repository for the move to the nxplain-sh organisation, and adds the tooling a first release needs.

Changes

• Repository move. Module path, internal imports and documentation links now point at github.com/nxplain-sh/protui. The old path keeps working via GitHub's redirect, but new tags should be cut under the new one.
• Releases. Adds .goreleaser.yaml and a tag-triggered release workflow that publishes macOS and Linux archives for amd64 and arm64, with version injected into protui --version. No Homebrew tap yet; that needs a nxplain-sh/homebrew-tap repository first.
• Code of conduct. Adds Contributor Covenant 2.1 with the maintainer's noreply address as the enforcement contact.
• Git hooks. Replaces the pre-commit framework with .githooks/pre-commit, installed by make hooks (core.hooksPath). Same checks, same Makefile targets, same staged-file scoping, one fewer tool to install.
• Housekeeping. Renames .golangci.yml to .golangci.yaml, and runs the prettier check in CI through make format-check so each check is defined once.

Verification

• make check passes.
• The new hook was exercised directly: all files staged runs all five checks; a Markdown-only commit runs prettier alone.
• goreleaser check validates the release config.
• No release has been tagged, so the release workflow is untested end to end.