docs: add a placeholder landing page for GitHub Pages

[refeed]

A placeholder landing page, living in the repo at docs/index.html, as a stopgap until the designer rebuilds it.

Pages is enabled on this repo but has never served anything: the gh-pages branch contains only .nojekyll, so https://stackguardian.github.io/tirith/ currently returns 404. This would be the first content that URL has served.

⚠️ Merging this does not publish it

Serving from the repo tree needs a one-time settings change, since Pages is currently pointed at the empty gh-pages branch:

Settings → Pages → Build and deployment → Source: Deploy from a branch → Branch: main / folder: /docs

I have deliberately not made that change — it is a production setting on a public repo, and flipping it before this merges would achieve nothing anyway. Say the word and I will, or it is two clicks.

Note the sequencing: the branch selected there must be one that contains docs/index.html. Based on feat/gate-capable-engine, so if Pages is pointed at main the page appears when #272 lands.

docs/.nojekyll keeps Jekyll away from the .md and .gif files already in that folder.

What it is

Six sections, in the order a stranger needs them: what it is and how to install it, the problem it solves, the two lines you add, what you get, what it works with, and platform mode last.

Deliberately plain — one hand-written HTML file, no site generator, no build step, no JavaScript, about 200 lines including roughly 50 of CSS. The repo has no site tooling and a placeholder is a poor reason to introduce some. Every prose block sits in its own commented <section>, so the copy can be lifted out without reading the markup. Light and dark via prefers-color-scheme; one column; no images, no fonts, no external requests.

Copy

Derived from README.md rather than newly written, so there is one source of truth to keep correct. A comment at the top of the file says so.

Claims are limited to what I could verify in the code:

• No OPA. Nothing in src/ references opa or rego, so it is absent from the page.
• No approvals. counts.approval_required exists, but platform/cli.py reports it as a warning that explicitly does not block — too weak to sell as a feature.
• Checkov is scoped to platform mode, because Tirith does not run it; report.py only renders findings the platform returns.
• Cost (providers/infracost), masking, the exit-code split and the bundle keeping documents next to code/ are all verified against the source.

No testimonials, no logos, no metrics, no signup form.

The @v2 problem, again

The one example on the page uses @v2, which does not exist — the newest tag on the action is v1.0.0-beta. This matches both merged READMEs rather than diverging from them, so cutting that one tag makes all three surfaces correct at once. It is worth doing before this page is publicly linked, since a landing page is the first thing a stranger copy-pastes.

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