feat: Implement Bluesky card previews without additional dependencies

[Copilot]

When a post contains URLs but no images, the Bluesky strategy now automatically fetches Open Graph metadata from the first URL and creates an embed card (app.bsky.embed.external). Thumbnail images are fetched and uploaded when available. Failures are silently ignored so posts still succeed without card previews.

Implements the same feature as #162 but without adding any new dependencies — uses a regex-based HTML parser instead of metascraper.

Changes

• src/strategies/bluesky.js:

  • Import BLUESKY_URL_FACET constant (already exported from bluesky-facets.js)
  • Add parseOpenGraphData(html) — extracts og:title, og:description, og:image from HTML via regex; falls back to the <title> element when og:title is absent
  • Add fetchOpenGraphData(url, signal) — fetches a URL and returns parsed OG metadata
  • Add createCardEmbed(options, session, facets, signal) — builds an app.bsky.embed.external object from the first URL facet, uploading thumbnail when present
  • In postMessage: add else branch to auto-generate card embed when no images are provided

• tests/strategies/bluesky.test.js:

  • Add externalServer MockServer for https://example.com and register with FetchMocker
  • Add describe("post with auto card preview", ...) with 5 tests:
    • Basic card preview from URL with title and description
    • Card preview with thumbnail image upload
    • Graceful handling when OG fetch returns an error (no embed)
    • No card when post has no URLs
    • Card preview using <title> as fallback when og:title is not present

Relates to #161

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[nzakas]

@copilot Let's use <title> as a fallback if og:title is not present.

Also, double-check if an image is required for a card preview or not. If it's required, then we should not create a card preview if there is no image.

[Copilot]

@copilot Let's use <title> as a fallback if og:title is not present.

Also, double-check if an image is required for a card preview or not. If it's required, then we should not create a card previ...

Done in bc80408. <title> is now used as a fallback when og:title is absent.

Regarding images: per the AT Protocol lexicon for app.bsky.embed.external, the thumb field is optional — a card preview can be created without an image. The current implementation already handles this correctly: if og:image is missing or the image fetch fails, the card is still created without a thumbnail.