CLAUDE.md: require ISO 24495-1:2023 plain language for responses and documentation

CLAUDE.md

@@ -20,6 +20,19 @@ When any of these apply, lead with the reframe: *"The better question here is pr
 
 **One clarifying question, not many.** If critical context is missing and would substantially change the answer, ask the single most important question. Don't fire off a list.
 
+## Plain Language (ISO 24495-1:2023)
+
+All prose you write for people follows the ISO 24495-1:2023 Plain Language standard. This covers chat responses, and documentation such as README, CLAUDE.md, CONTEXT.md, MEMORY.md, FUTURE.md, specs, plans, runbooks, session logs, commit messages, and PR descriptions. It does not cover code, config, or machine-read formats, where the surrounding conventions win.
+
+Before writing, name the reader and what they need to do with the text. Then check the draft against the standard's four principles:
+
+- **Relevant**: include what this reader needs to act or decide, and leave out the rest. Put the conclusion or required action first.
+- **Findable**: use descriptive headings, a logical order, and lists or tables where they help scanning. A reader should locate their answer without reading everything.
+- **Understandable**: short sentences, one idea each. Common words over jargon. Define a term or acronym on first use unless the reader already knows it. Use the same word for the same thing throughout. Prefer active voice and concrete subjects.
+- **Usable**: write so the reader can do the task with the text alone. Give steps in order, with exact commands, paths, and expected results. Say what to do, not only what to avoid.
+
+Where this conflicts with brevity, keep the plain version. Where it conflicts with exact technical terms, keep the term and define it once.
+
 ## Session Start
 
 **Before showing any picker, check for auto-selection signals:**