fix(cli): allow blog and posts changelog slugs

[devin-ai-integration]

Description

Linear ticket: Refs

valid-changelog-slug errors on any changelog whose URL contains no allowlisted segment, on the grounds that its .rss/.atom/.json feeds would 404. A changelog published at /blog is exactly that case today, so authoring a blog on the changelog pipeline (fern-api/fern-platform#14055 adds layout: blog) fails fern check outright.

This adds blog, blogs, and posts to CHANGELOG_FEED_ALLOWED_SLUGS, keeping the rule in sync with the server-side list the docs bundle matches feed requests against (packages/commons/docs-server/src/patterns.ts in fern-platform, extended in the same way by that PR). Both lists must move together — allowing it here alone would validate a changelog whose feeds still 404.

Changes Made

• Add blog, blogs, posts to the rule's allowlist.
• Cover a top-level /blog changelog as valid in the rule's tests.
• Add an unreleased CLI changelog entry (type: fix).

Testing

• Unit tests added/updated — turbo run test --filter=@fern-api/docs-validator: 18 files, 207 tests passing.
• Manual testing completed — fern check on a docs project whose blog changelog resolves to /blog reports 0 errors (previously an error on the changelog slug).

Link to Devin session: https://app.devin.ai/sessions/da27e5183b724ee0953e0cd3f4cbcd06

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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