fix(bins): every npx bin was a silent no-op + add Astro integration (0.5.0)

[RelayStack]

The bug this fixes

Every bin in the published package was a silent no-op.

const isCli = process.argv[1] && import.meta.url.endsWith(process.argv[1].split("/").pop());

npm installs bins as node_modules/.bin/* symlinks. When npm (or npx, or npm run) executes one, process.argv[1] is the link (astro-seo-llms) while import.meta.url is the real file (llms-txt.mjs). The comparison is therefore always false, the CLI block never runs, and the process exits 0 having written nothing.

Reproduced on 0.4.0:

$ npx astro-seo-sitemap --dir src/content --site https://example.com
exit=0        # no output, no file

That means the README's advertised paths — npx astro-seo-llms …, build-script hooks, the github: shorthand — all did nothing for anyone who tried them. Fixed by resolving the path:

import.meta.url === pathToFileURL(realpathSync(process.argv[1])).href

Covered by a new test (bins run when invoked through a .bin-style symlink) that executes a bin through a symlink exactly as npm does.

What else is here

• astro-seo-toolkit/integration — a real Astro integration. Writes llms.txt, llms-full.txt, sitemap.xml, robots.txt during astro:build:done, from one or more content dirs (deduped by URL). Exports generate / writeFiles / normalizeOptions for build scripts.
• New bins: astro-seo-toolkit (all files at once) and astro-seo-check.
• Usage hints now print the npx astro-seo-* form instead of node <file>.mjs.
• scripts/sync-skill.mjs generates the skill's vendored copies and restores the executable bit; npm run check:skill fails on drift.
• README rewritten — who it's for, the integration, a comparison table, and install paths that were actually tested.
• One documented path removed: npm 12 defaults allow-git=none, so npx -y github:RelayStack/astro-seo-toolkit … errors out (EALLOWGIT), and even with --allow-git=all the bin isn't exposed. Replaced with a git clone snippet that works.
• CODE_OF_CONDUCT.md, CHANGELOG/CITATION 0.5.0, examples/astro on the integration, social card redesigned (docs/social-preview.png).

Verification

$ npm test          → 33/33 pass
$ npm run check:skill → in sync
$ npm pack && install tarball, then:
$ npx astro-seo-toolkit --dir content --site https://example.com --out dist
llms.txt, llms-full.txt, sitemap.xml, robots.txt written to dist (1 pages).
$ npx astro-seo-check --dir dist --site https://example.com
{ "ok": true, "issues": [] }

Held back

.github/workflows/test.yml (adds the check:skill step and a packaging smoke test that installs the tarball and invokes each bin) and a new .github/workflows/release.yml are not in this PR: the token used to push lacks the workflow scope, so GitHub rejects them. Run gh auth refresh -h github.com -s workflow and they can go in a follow-up.

Follow-up (not in this PR)

Publish 0.5.0 to npm so npx astro-seo-toolkit resolves from the registry. Requires npm login.

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