docs: Normalize docs to current repo name and package manager

[sm86]

Problem

Several docs still tell people to clone ekai-gateway and use npm, while CONTRIBUTING.md:7 documents a pnpm workspace flow. The repo is now ekailabs/contexto.

Starting points

• docs/getting-started.md:21
• docs/USAGE_WITH_CODEX.md:17
• docs/USAGE_WITH_CLAUDE_CODE.md:17

Other affected files: docs/ROFL_DEPLOYMENT.md, docs/intro.md (badge), docs/memory-plugin.md (npm install @ekai/memory).

Goal

• Replace git clone .../ekai-gateway with .../contexto and update cd paths.
• Switch npm install / npm run X examples to pnpm install / pnpm run X (and pnpm add / pnpm --filter).
• Leave alone (separate scope): the Ekai Gateway product name in prose, the ghcr.io/ekailabs/ekai-gateway published image, the local ekai-gateway docker tag, and the ekai-gateway-runtime compose stage.

Out of scope

• Root package.json is still named ekai-gateway — rename has wider blast radius (release flow), worth a separate ticket.
• Whether to fully rebrand "Ekai Gateway" → "Contexto" in product copy.