docs: clarify Facility introduction and setup path

[alvaroloes]

What changes

The README now introduces Facility through the work a reader will do: start a story, continue with an agent, inspect the running result, review through GitHub, and retain the workspace. The existing product tagline leads the page, and the quick start moves ahead of detailed reference material.

Setup commands remain intact. The overview links to the maintained guides for manifests, MCP, lifecycle, security, and operations instead of repeating their reference content. It also distinguishes self-hosted Facility infrastructure from the model services used by Claude Code and Codex, and explains maintainer access and budget preflight limits.

The documentation contract test now protects the core concepts and setup/security links without requiring the old introduction and section titles verbatim.

Why

A first-time reader had to pass a long feature inventory and configuration examples before reaching setup. Some wording also implied that self-hosting keeps model traffic inside the deployment. The shorter introduction gives readers a concrete path to try the product and find its operating boundaries.

Verification

• pnpm verify passes locally, including lint, typecheck, clean build, critical integration tests, remaining tests, guards, and the configured dependency audit.
• Documentation verified beyond the tests: checked lifecycle and archive behavior against the implementation, reviewed the permission/model-provider claims, and retained the setup command blocks.
• Documentation updated.

Also ran pnpm --filter @facility/docs test (11 passing), pnpm --filter @facility/docs build, and the Markdown link guard. The dependency audit reports two high advisories already covered by the repository's existing ignore policy; this PR changes no dependencies. A fresh provider-backed installation was not performed for this documentation change.

This is a user-visible, non-breaking documentation change. It changes no runtime behavior, permissions, persistence, schema, costs, analytics, or mirror operations, and needs no migration.

Review note: #329 also edits README installation instructions. This PR preserves those commands so that change can be reviewed separately; check both when merging.