write-docs

Invocation points:

• User asks to write or rewrite a piece of documentation
• Before shipping a feature that adds user-facing behavior — docs need updates
• After write-milestone-brief when the brief needs a public-facing summary for non-GSD consumers
• PR description for a meaningful change

<core_principle> WRITE FOR THE FRESH READER. The test is not "does the maintainer understand it" — you are the maintainer. The test is "can someone who has never seen this code act correctly after reading this?"

ITERATE STRUCTURE BEFORE PROSE. An outline that's wrong will not be saved by good sentences. Agree on the sections, what each is for, and in what order before drafting the body.