refactor: drop dead _BUILD_ARG_ fallbacks from option variable reads

[baxyz]

Summary

• claude-dev, mistral-dev, cline-dev, copilot-dev, github-dev, and dotfiles-sync each read their feature options as "${_BUILD_ARG_X:-${X:-default}}" instead of the plain "${X:-default}" every other feature in this repo already uses.
• That prefix is only ever set by the devcontainers CLI for the legacy internalVersion: "1" manifest shape (confirmed via nub's own install.sh comment, and confirmed no feature in this repo declares that field) — so the _BUILD_ARG_ branch never fires and always falls through to the plain env var anyway.
• Pure dead code removal, not a behavior change: verified the old and new forms resolve identically for both a set and an unset env var.
• cline-dev and copilot-dev had no ## Version History section at all — added one alongside this change.

Board card: "Unify feature option variable format (legacy BUILD_ARG* vs modern no-prefix)" (Backlog, Medium priority).

Test plan

• bash -n syntax checks, JSON validated on all 6 features
• Confirmed zero remaining _BUILD_ARG_ usages outside of nub's explanatory comment
• Confirmed old vs. new form resolve identically (set and unset cases)
• CI

🤖 Generated with Claude Code

[github-actions]

✅ PR Validation Passed

All checks passed!

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📋 Pipeline Status

	Job	Status
✅	🧾 Conventional Commits	passing
✅	🔖 Version Bump	passing
✅	🧪 Feature Tests	passing
✅	🐚 ShellCheck	passing

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_{🤖 Generated by @helpers4 CI • 2026-09-08}