fix: avoid redundant AI extension updates

[CollieIsCute]

Summary

• check npm releases and the shared skills revision in parallel on every full apply
• run Claude, Codex, and OpenCode updates only when their installed version is stale or missing
• stage and validate OpenCode cache replacements so failed or concurrent updates preserve the known-good cache
• replace the templated inventory with direct declarations and run sync regression checks in Linux/macOS CI

Verification

• bash tests/sync-ai-extensions.sh
• chezmoi verify
• chezmoi diff --exclude=scripts is empty
• opencode debug config
• claude doctor
• repeated no-change chezmoi apply -v: 0.636s