feat(astro-seo-graph): add indexNowOnBranch for branch-gated IndexNow submission

[gregturn]

Problem

The indexNow integration submits IndexNow submissions for every branch unconditionally. This means that platforms that deploy preview branches (Cloudflare Pages, Vercel, Netlify), will submit preview branches to IndexNow using the same key as production — URLs the production host hasn't served yet. That wastes daily quota and risks getting the key flagged invalid, forcing a rotation.

There's no built-in way to conditionalize submission; users either accept the unconditional behaviour or write their own branch-detection wrapper, which is what prompted this patch.

What This Adds

indexNowOnBranch(branch, options, productionBranch?) — a thin helper exported from @jdevalk/astro-seo-graph/integration (and re-exported from the main entry).

It passes IndexNowIntegrationOptions through unchanged when branch matches productionBranch (default "main") and returns undefined otherwise, so seoGraph() skips submission entirely on non-production deploys.

Here is an example of usage (CloudFlare-specific):

// astro.config.mjs
import { defineConfig } from 'astro/config';
import seoGraph, { indexNowOnBranch } from '@jdevalk/astro-seo-graph/integration';

export default defineConfig({
    integrations: [
        seoGraph({
            indexNow: indexNowOnBranch(process.env.CF_PAGES_BRANCH ?? '', {
                key: process.env.INDEXNOW_KEY,
                host: 'example.com',
                siteUrl: 'https://example.com',
            }),
        }),
    ],
});

NOTE: Existing configs that pass IndexNowIntegrationOptions directly are unaffected, making this a NON-BREAKING change.

Documentation is added to the existing IndexNow sections.

Verification

Test suite expanded with 4 new cases covering indexNowOnBranch