Clarify that an action's result is flat (res.field), not res.data.field

[shivamsn97]

An @action's awaited result on the client is the flat { ok, ...yourReturn } — your returned fields sit directly on it (res.title), and a successful result has no .data. But useAction also exposes a separate .data property (the last successful return), and it's easy — for a person or an AI coding agent — to conflate the two and write res.data.title. On success that's undefined, so it silently no-ops inside if (res.ok) and the mutation looks like it did nothing until the page reloads.

The runtime and the shipped TypeScript types were already correct — the prose was the weak link. This makes the contract explicit everywhere it's taught:

• Scaffold AGENTS.md — the result-shape rule, the useAction example, the toolkit reference, and a new DON'T all state that fields are top-level (res.x) and warn against res.data.x.
• Server Actions guide — a note disambiguating the awaited result from the hook's .data property.
• Client API reference — the same note beside the return-value table.
• README — the inline comment drops an ambiguous ...data spread.
• Changelog — entry under Unreleased.

No behavior change. A fresh pyxle init emits the updated guidance and the CLI/scaffold suite stays green.