docs: install, init and upgrade guides for beta.1

[pacphi]

Why

The guides describe global installs and setup (ledger section E).

In scope

README install block and upgrade steps; rewrite docs/installation.md and docs/codex-statusline.md; replace docs/setup.md with an init guide; add "Upgrading from the user-level versions" to docs/upgrading.md.

Out of scope

The full docs pass (#383).

Acceptance criteria

• A new user can follow the guides with npx @pacphi/agentic-kit@beta init
• No guide tells users to npm install -g

Dependencies

Blocked by: #331, #336

Blocks: #383, #385

Plan

• Release: 4.0.0-beta.1
• Size: size:M · Workstream: ws:ga-readiness
• Epic: feat: project-scope P1, project layout and ak init (epic) #296
• Card: P1-09 in the card inventory
• Sources: Supersession ledger (docs(plans): v4 GA master plan, supersession ledger and v5 registers #290) section E; Design (docs(plans): project-scoped management design and its prerequisites #289) The instructions

Verification

• pnpm run lint:md