cli: retire `os create example` — it is a weaker `os init` plus a README; docs point at `os init` (item 2 of #15531)

[os-zhuang]

Filed by the director seat under the #15531 ruling (decision batch #66, 2026-09-07, option B — do not merge the command families; fix the measured problems).

Why

#15531 rendered the real emission of os init and os create and hashed it: the only template-level duplication between the two families is os create example, which emits a subset of os init's project (plus one README). The two families' emission policy is already unified through four shared exports (PR #15974), so the remaining duplicate is this one template.

Scope

• Remove the example template from os create; os create keeps plugin (kernel code plugin) and its other non-init shapes.
• Docs: every page that mentions os create example points at os init instead; the "Which scaffolder?" hint is corrected in the sibling card (plugin wording), so coordinate wording with it.
• Published CLI surface removed ⇒ Clause-② yes: landing PR carries needs:contract-review; changeset (minor/major per the CLI's versioning policy) states the removal; no alias, no deprecation window (maintainer 2026-08-27).
• Pin: os create example exits non-zero with a message naming os init; docs-parity pin updated.

Acceptance

• template gone, refusal message tested
• docs updated with the docs-parity pin green
• changeset present

Refs #15531, PR #15974.