Verify OpenSpec docs

Check finished docs prose against reality. The point of a fresh context is that the reviewer hasn't watched the prose get written, so it can't be talked into the author's assumptions.

This skill runs only when the user asks for it. Drafting is owned by write-openspec-docs; don't invoke this from inside a drafting session unless the user requests a verification pass.

Scope the run

1. Confirm the target: a page, one ## section, or a list of changed claims. If invoked without a target, ask.
2. Read the README at the root of the docs tree the target lives in; its invariants and page map are part of what gets checked.
3. One subagent per unit (one ## section, or the stated claim list). A full page is several subagents, run in parallel.

Spawn the reviewer

General-purpose subagent. Subagents don't inherit skills, so the prompt hands the reviewer everything by path. Fill every placeholder, make every path absolute, and send:

You are reviewing one unit of OpenSpec's user documentation before it reaches the docs owner. Be the two hardest readers it will meet: a skeptical developer reading it cold, and a fact-checker with the repo open.