docs: full documentation pass for GA #383

Why

Remaining guides describe the user-level line (ledger section E).

In scope

README rewrite, troubleshooting.md, devcontainers.md, docs/README.md index, docs/ddd pages and the explainer's three-scopes section.

Out of scope

v5 docs.

Acceptance criteria

• No guide describes user-level management as current

Dependencies

Blocked by: #338, #349, #363, #377

Blocks: #389

Plan

• Release: 4.0.0-rc.1
• Size: size:M · Workstream: ws:ga-readiness
• Epic: docs: 4.0 GA readiness (epic) #303
• Card: GA-04 in the card inventory
• Sources: Supersession ledger (docs(plans): v4 GA master plan, supersession ledger and v5 registers #290) section E

Verification

• pnpm run lint:md