docs: add stable 0.91.0 changelog

[zxch3n]

Related issue

Refs #350

Problem / pressure

The stable 0.91.0 release needs a bilingual public changelog that covers the user-visible OSS changes and credits the external Kimi contribution included through the managed runtime.

Summary

• Add equivalent English and Chinese changelog entries dated 2026-09-05.
• Link the relevant public Lody and ACP pull requests.
• Credit @nokizw for the Kimi provider display and search contribution.

Before / after

Before	After
The public site had no changelog entry for the 0.91.0 stable release.	English and Chinese entries describe the reviewed release scope and contributor credit.

Test plan

• git diff --check passed.
• Reviewed the committed diff and verified that only the two changelog MDX files changed.
• Product tests and the site build were deliberately skipped because this release step changes content only and the release operator requested no test run.

Context handoff

Instructions for reviewing agents

• Review focus: Compare the English and Chinese MDX entries for equivalent meaning, matching PR links, version, date, and contributor credit.
• Decisions to challenge: Confirm that the grouped user-facing summaries accurately represent the linked public changes without exposing internal security details.
• Plausible failures / evidence gaps: A link, translation, or grouped summary could misattribute a change; no site build was run for this content-only PR.

Authoring context

• User goal / directives: Prepare the next stable Lody release, mention the included public pull requests, and explicitly credit the external Kimi contributor while omitting the Codex upgrade item.
• Constraints / non-goals: Change only the bilingual public changelog; do not alter product code, generated files, versions, or runtime pointers.
• Risk-bearing decisions: The candidate version is 0.91.0 based on the stable Release Please rules and remains subject to the generated release PR.
• Destructive or irreversible behavior: The change only adds two reviewable MDX files and performs no deletion, migration, publication, or production merge.
• Deliberately not done or tested: Product tests and the site build were skipped by request; diff integrity and exact file scope were checked locally.
• Unknowns / confidence: Confidence is high in the reviewed content and links; Release Please remains authoritative for the final version.