[David Pine (IEvangelist)]

Summary

Adds a Prompt tab to the OG Viewer canvas and an agent-callable get_prompt action. Both turn a URL into one ready-to-use prompt for an AI assistant: a short context block, composable task modules (audit, fix head tags, rewrite copy, share image, structured data, agent readiness, implement, verify), the user's own notes, and a versioned JSON snapshot of the page's SEO and social metadata (URL and redirects, HTTP headers, title, description, canonical, robots, hreflang, JSON-LD, Open Graph, Twitter, diagnostics, agent readiness).

Stacked on #1918. This branch is based on #1918's branch, so the diff shows only the composer commit. I'll retarget it to main and rebase once #1918 merges.

How it works:

• One pure module, lib/build-prompt.mjs, builds the payload and the prompt. The extension server and the canvas UI both use it, so the tab and the action can't drift.
• The tab has Copy prompt, Copy JSON, Open in Copilot, and Create issue (the last two appear when the page links to its source repository). The fifth tab also gets a responsive toolbar fix.
• Page content is untrusted. The JSON is fenced and followed by a reminder, the Page: line carries only the requested URL, and redirect targets, the canonical URL, and the detected repository appear only inside the JSON.
• The payload is bounded. Strings, lists, and JSON-LD have fixed limits, and anything cut is named in truncated; JSON-LD that is left out is flagged dataOmitted. The limits are listed in the README.

Known limit: a page that pushes every limit at once can still produce a prompt larger than the 512 KB request limit used by Open in Copilot and Create issue, and GitHub caps an issue body at 65,536 characters. Copying always works.

Third-party links and affiliations

None. The tests use reserved .example hostnames as hostile fixtures; no real third-party links are added.

Validation

• node --check on extension.mjs, lib/parse-og.mjs, and lib/build-prompt.mjs: all pass.
• node --test ".github/extensions/og-preview/tests/*.test.mjs": 72 pass, 0 fail.
• Control: against the lib files from before the review fixes, 11 of the new and updated tests fail, so the tests catch the problems they target.
• Live check in a real browser with the extension reloaded, on https://aspire.dev/reference/api/csharp/ (the page Fix OG Viewer failing to load pages over the 6 MiB fetch cap #1918 fixes):
  • Browse shows the whole page (6.45M characters, 4,996 package links, footer).
  • The Prompt tab shows all eight modules, the size readout (15,868 characters, about 3,967 tokens), the truncation notice, and a valid JSON snapshot with the agent-readiness checks joined in.
  • get_prompt returns the same shape; the Page: line shows only the requested URL and the detected repository appears only in the JSON.
• UI, CSS, and the agent-readiness path aren't covered by CI; the live check above is the only coverage for them. The design linter still reports 3 findings in styles.css (side-tab, bounce-easing, layout-transition) that were there before this change.
• CI: OG preview tests runs on this PR. ci.yml only runs for PRs into main or release/*, so it will run after the retarget.