docs: add a GitHub Pages documentation site under docs/

[lavallee]

Adds a five-page documentation site for Spindle, built as an artoo artifact and styled with the vendored artoo-kit design system. Served by GitHub Pages directly from /docs — no build step.

Spindle "adds an extra step or two" to a skill workflow, so this site leads with the rationale (why compose skills at all) and then explains how it works, how it's evaluated, and how to use it — for humans and for agents.

Pages (docs/)

• index — overview, two-minute quickstart, concept map
• rationale — the case against the skill pile (context budget, incoherent "pidgin" blends, one-size-fits-all bloat, wrong-surface phrasing, guardrail erosion) and why the extra bind step pays for itself
• how-it-works — the data model and the six-stage bind pipeline, rendering profiles, doctrine, the safety floor, determinism/caching
• evaluation — availability vs improvement, paired held-out cases, the argv runner contract, receipts
• guide — install, bind, build your own packages/distributions/channels/profiles, and the env-var seams for extending
• agent surface — llms.txt index + a single-file bundle.md for paste-in LLM context

Also

• Records the artifact in artifact.toml ([build] site = "docs", github-pages docs mode)
• Replaces the old docs/ placeholder page (keeps docs/skill-evaluations.md)
• Retrofits README.md to lead with the rationale and link to the site + agent surface
• Adds a CHANGELOG entry

To publish

Set Settings → Pages → Source → Deploy from a branch, main / docs. The site publishes at https://lavallee.github.io/spindle/.

Verified in a headless browser: correct article grid + margin-note gutter + pipeline, dark/light both work, zero console errors, no horizontal overflow at desktop or mobile widths. The existing test suite is unaffected (the private-surface scan passes).

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