feat(web): idx website and user docs (Astro Starlight)

Landing page plus docs for install, quickstart, config, the stocks and
ownership CLI, output and exit codes, the MCP server and per-agent setup,
agent usage, and an agent-led install page (agents/install.md) that a
coding agent follows from a one-line prompt.

- Every docs page is also served as Markdown at <path>.md; llms.txt and
  llms-full.txt come from starlight-llms-txt; a SKILL.md for skill-based
  harnesses is under /skills/idx/.
- The CLI reference and MCP tools pages are generated from idx --help and
  the tools golden; a new web-generated flake check fails when they drift.
- nix develop .#web provides bun; Cloudflare Pages builds web/ on pushes.
- Package homepage metadata points at https://idx.rasyidanaf.com.
- .cloudflare/ (the cf CLI's account cache) is git-ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/components/PageTitle.astro

@@ -0,0 +1,34 @@
+---
+import Default from '@astrojs/starlight/components/PageTitle.astro';
+
+const { id } = Astro.locals.starlightRoute.entry;
+const markdownPath = `/${id === 'index' ? 'index' : id}.md`;
+---
+
+<div class="title-row">
+	<Default />
+	<a class="md-link" href={markdownPath} title="This page as Markdown, for agents">Markdown</a>
+</div>
+
+<style>
+	.title-row {
+		display: flex;
+		flex-wrap: wrap;
+		align-items: baseline;
+		justify-content: space-between;
+		gap: 0.5rem 1rem;
+	}
+	.md-link {
+		font-family: var(--sl-font-mono);
+		font-size: var(--sl-text-xs);
+		color: var(--sl-color-gray-3);
+		border: 1px solid var(--sl-color-gray-5);
+		border-radius: 0.375rem;
+		padding: 0.25rem 0.5rem;
+		text-decoration: none;
+	}
+	.md-link:hover {
+		color: var(--sl-color-white);
+		border-color: var(--sl-color-gray-4);
+	}
+</style>