Add skill: serradura/okf

[serradura]

Adds the okf skill to Context Engineering. I maintain it, so this is a self-submission.

The skill teaches an agent the Open Knowledge Format: project knowledge kept as a folder of Markdown files with YAML frontmatter, one concept per file, links between files forming the graph. It covers authoring a bundle from existing code and docs, migrating docs into one without rewriting them, keeping it in sync as the code changes, and reading from it to answer a question instead of re-reading the codebase.

It sits next to sametbrr/llm-wiki-manager and Tubo2333/obsidian-knowledge-brain in this section, with one difference worth naming: the skill is paired with a CLI that checks its output. The agent's curation gets verified by okf validate for spec conformance and okf lint for curation quality, so the knowledge base has a deterministic gate rather than relying on the model's judgment about its own work.

Against the requirements in CONTRIBUTING:

• Public repo, Apache-2.0, with a SKILL.md plus playbooks, reference material, and templates rather than a single file.
• Not new. The gem is at 1.10.0, roughly 5.9k downloads on RubyGems, and the repo is at 78 stars.
• Portable across agents. okf skill .claude or okf skill .agents installs the same skill anywhere that reads the format.
• Description is 10 words, and the entry goes at the end of the category.