feat(posts): add EP71 ChatSDK podcast episode

[AnnatarHe]

Motivation

• Add a new published episode entry for EP71 following the existing MDX frontmatter and shownotes structure used by other episodes.
• Provide canonical links for Xiaoyuzhou, YouTube, and Bilibili and include fields required by the content schema so the episode is discoverable in feeds and site metadata.
• Normalize categories to lowercase and include a concise subtitle/excerpt describing ChatSDK as a unified conversation-integration layer for AI agents.

Description

• Create src/content/posts/ep71.mdx with the requested frontmatter fields including type: podcast-episode, status: published, slug: /posts/ep71, guid: 71, title, author, season, episodeNumber, publicationDate: 2026-07-16 14:00:00, url/xyzLink/youtubeId/biliUrl, and other metadata.
• Set subtitle and excerpt to describe ChatSDK as a unified conversation-integration layer and normalize the supplied hashtags into lowercase categories.
• Add Chinese shownotes under ### 📕 Shownotes with an MDX-compatible list for the “视频里会带你看” items and preserve the AsyncTalk closing line.
• Commit the new file with the message feat(posts): add EP71 ChatSDK podcast episode.

Testing

• Ran pnpm astro check, which completed with 0 errors and no blocking warnings (only informational hints).
• Ran pnpm build, which completed successfully and generated the static site output.
• Verified git diff --check and git status --short to ensure the working tree is clean after committing the new episode file.

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Codex Task