docs(changelog): add new entries from changelog

[windmill-internal-app]

This PR adds new changelog entries.

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Summary by cubic

Adds a new sidebar changelog entry: “Reasoning effort for AI agent steps” (2026-07-04) linking to https://www.windmill.dev/changelog/ai-agent-reasoning-effort. Keeps the in-app changelog current.

^{Written for commit 15c166d. Summary will update on new commits.}

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[github-actions]

Pi Review

Good to merge.

This is a docs/config-only change — a single new entry added to the static changelogs array in frontend/src/lib/components/sidebar/changelogs.ts. The entry follows the existing Changelog type, is well-formed, and the date matches the day of the commit. No code logic, new public surfaces, or in-app behavior was introduced.

Test coverage: CI / workflow / docs / config-only — no automated tests expected. No manual verification needed; the changelog entry will be picked up by the sidebar changelog component on next render.

[github-actions]

Codex Review

cc @windmill-internal-app[bot]
Mergeable, but should ideally address nits: broken changelog link

Found 1 issue:

1. [P2] frontend/src/lib/components/sidebar/changelogs.ts:10 points the new sidebar entry at a changelog URL that currently returns HTTP 404. I checked neighboring changelog links and they return 200, so this looks specific to the new ai-agent-reasoning-effort entry. Users clicking “Reasoning effort for AI agent steps” from the sidebar would land on a missing page unless the changelog page is published or the slug is corrected.

Test coverage

No automated tests are expected for this static frontend changelog data update.

Manual verification before merge: open the changelog/sidebar entry for “Reasoning effort for AI agent steps” and confirm it navigates to a published changelog page rather than a 404.