Drop "first" from the full-build checklist copy

[juanmaguitar]

Stacked on #111 — merge that first; this shows only the copy diff.

Now that the trunk update chain rebuilds automatically, "Run first full build" reads as if later builds were also the user's job. The one-time framing is already carried by the "Initial setup checklist" heading, so the copy drops "first"/"once"/"initial" and says explicitly where later builds come from:

• Step title: "Run first full build" → "Run full build"
• Description: "Compile WordPress Core once to generate the initial dist files." → "Compile WordPress Core to generate the dist files. Later updates rebuild automatically."
• Button: "First build complete" → "Build complete"
• Dev-server gate alert: "complete the first full build" → "complete the full build"

Copy-only; no logic changes. Stacked on the update-path branch because the new description's claim ("later updates rebuild automatically") is only true with #111's changes.

🤖 Generated with Claude Code

[github-actions]

0 findings across the five dimensions (architecture, security, performance, cross-platform, tests).

This PR only changes UI copy - three strings ("Run first full build" -> "Run full build", the matching button label, and the alert text) - in src/renderer/index.jsx and its committed esbuild bundle src/renderer/index.js. No logic, IPC surface, spawn behavior, persistence shape, or platform-specific code is touched, and the two files are edited consistently with each other. Nothing to flag.