docs(changelog): release v0.26.0

Rolls [Unreleased] into [0.26.0] - 2026-10-11 and leaves a fresh empty
[Unreleased] on top. Adds the mandatory headline (documents mxcli writes
survive Studio Pro's merge and diff) and grouped entries for the
user-facing changes since v0.25.0 that had none: the companion
mod, mxcli fix property-sets, property-set completion, DateTime not
localized, the describe -> exec control-flow fixes, the wider check -p
predictions and the language poster. Adds a v0.26.0 release post to the
docs-site blog, with the poster image linking to the interactive version.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014DfG9NpcA979LzU5R8j1Ay

Author: ako