[Srushti Giridhar Borate (srushti-learner)]

Summary

Update the README current release to v0.15.21 and the previous-release table to v0.15.20 (2026-10-20). Switch all current and previous installer links to the preview-specific aka.ms aliases, matching the current aliases introduced in #272.

Type of change

• New agent (under agents/<name>/)
• Update to an existing agent
• New starter kit (under starter-kits/<name>/)
• Update to an existing starter kit
• Schema change (docs/schemas/**)
• Workflow / script change (.github/**)
• Documentation only
• Other (describe below)

Related issue / tracking

Companion to #272. Follows the README-only release update in #247; #245 was the corresponding previous manifest update.

Schema impact

• No schema changes
• Schema changes are backward compatible
• Schema changes are breaking — add the breaking-change label and link the rollout plan below

Validation checklist

• Documentation (README.md, schemas) is updated to match the change.
• No hand-edits to .auto-registry/** (it is generated post-merge).
• No OS / editor artefacts committed (.DS_Store, .env, *.swp, .idea/, .vs/, etc.).
• If model-weight files are added, they are Git-LFS tracked, ≤5 GB each, and in an allowed format.
• No secrets, customer data, or internal-only URLs are committed.
• All Markdown links resolve.

Validation:

• git diff --check - passed.
• curl --silent --show-error --head --max-time 20 https://aka.ms/discovery/download/preview/{x64,arm64,osx,rhel}/{current,previous} (each of the eight URLs checked separately) - all returned HTTP 301, but their destinations are outdated: current points to v0.15.19 and previous points to v0.15.18.
• python -m pytest .github/tests/ -v - not run; this PR changes only release text and download links, not validator code.
• Other unchanged README links were not revalidated.

Reviewer notes (optional)

Keep this PR in draft until the preview aka.ms aliases are repointed: current must serve v0.15.21 and previous must serve v0.15.20. Verify installer filenames against the published assets at that time. This PR edits link references only; it does not change aka.ms redirect configuration or the manifest handled by #272.

The previous-release date is taken from the current GitHub Release's publishedAt timestamp, 2026-10-20T14:11:47Z.

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