feat: env vars article cross-links, content calendar revision, agent skills

[lmammino]

Summary

Follow-up to #38 (merged before these changes landed on the branch).

• Cross-links: HTTP request guide and Docker guide now link to the env vars article; the env vars article links out to the path traversal, HTTP request, and reading/writing files guides (NODE_USE_ENV_PROXY, UV_THREADPOOL_SIZE, and .env security anchors).
• Content calendar: env vars marked complete, existing-content table updated with the two off-calendar articles (Node.js 26, release schedule), internal linking map updated to the real link graph, next-up recommendation (CLI article Fix alignment of praises #4 before SQLite Update Buy links #3), promotion notes.
• Agent skills: adds .agents/ marketing skills, .claude/skills symlinks, and skills-lock.json.

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