fix(docs): fix docs deployment domain

Summary

• Point the docs Cloudflare Worker route at docs.buildonce.dev.
• Update the GitHub deployment environment URL to https://docs.buildonce.dev.
• Align the workflow Wrangler version with the docs package pin so the deploy action reuses wrangler@4.100.0 instead of trying to install 4.90.1.
• Add a cheap once workflow change detector so docs-only pull requests skip Rust, release-build, dogfood, shellspec, and Windows jobs.
• Document that PR titles should use Conventional Commit style without a [codex] prefix.

Root Cause

Docs builds were passing, but deployment failed in cloudflare/wrangler-action because the workflow requested wranglerVersion: "4.90.1" while the project had wrangler@4.100.0 installed. The action attempted an npm install of the older version and npm failed before deployment.

The once workflow also ran unconditionally on every pull request, so docs-only changes still triggered release-mode Rust builds and the downstream dogfood jobs.

Validation

• mise exec -- aube ci
• mise exec -- aube run build
• mise exec -- aube exec -- wrangler --version
• mise exec -- aube exec -- wrangler deploy --dry-run
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/once.yml"); puts "yaml ok"'

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