Add coding-agent token-saving analysis

[Yohjisakamoto]

Summary

Add the freely available text article “Token-Saving Plugins Are Mostly Stupid Idea” to AI → Vibe Coding and Spec-Driven Development.

The article evaluates coding-agent token-saving claims against matched long-horizon repository-rewrite runs. It publishes per-run data, a computed summary, methodology, a 293-round activation audit, and a broader 140-run token ledger, while explicitly treating the n=2 plugin arms as insufficient for a causal claim.

Contribution-guideline checks

• Text article, not a video.
• Freely available from its author; no pirated material.
• Placed in the relevant section and in lexicographic order.
• One resource in this pull request, using the required link format with no trailing slash.
• Checked the README and existing pull requests/issues; no duplicate was found.
• git diff --check passes.

Affiliation disclosure

I am affiliated with Tura, which published the article and its accompanying benchmark data.