fix(apis): keep Arcmira description within 300 characters

[zealous1]

Submission overview

The Arcmira entry merged in #67 still has a 394-character description. The validation report on that PR rejects descriptions above 300 characters. Shorten it to 251 characters and regenerate the affected directory outputs.

• Resource: Arcmira: YouTube Transcript Search
• Source: shelf/apis.json
• URL: https://arcmira.com/docs
• Pricing: permanent Free tier with separate paid features, stated in the description

Validation

• Official validate-shelf.ts: passed all 524 entries.
• Official validate-pr.ts against origin/main: passed 98 API entries, no schema errors or duplicate collisions.
• Official check-links.ts in an isolated one-entry fixture: Arcmira documentation returned HTTP 200. This was a scoped link check, not a full catalog audit.
• Ran build-site.ts, build-readme.ts and generate-seo.ts; kept only the Arcmira output changes and preserved existing generation timestamps. CSS is unchanged.
• git diff --check: passed.

Scripts ran through an existing installed tsx executable, with no dependency or lockfile changes. The main URL, title, category, Free-tier limits and all other entries are unchanged. I maintain Arcmira.