docs: add distribution strategy page with three doors

[PeterGuy326]

What

• New docs/DISTRIBUTION.md: the distribution strategy for doc (and mem, which follows
  the same doors). It defines the three supported paths — local single-player (current
  default, honestly labeled), ByteFolk-hosted SaaS (gated behind demand signals plus
  multi-tenant/privacy/paid-infra prerequisites), and guided self-hosting (current main
  path) — with per-door identity, cost, and compliance boundaries.
• The page also records the authentication consistency principle (one protocol across all
  deployments, per-deployment client sessions; SaaS is just a preset deployment address),
  the guidance surfaces (doc init routing question, README/landing three doors,
  doc doctor door report), operator security boundaries, and migration between doors as
  a product promise per GOAL.md north star 5.
• README: link the new page next to GOAL/SPEC.
• CHANGELOG: [Unreleased] Added entry.

Why

The core value (person-to-person sharing) only holds when a deployment is reachable by
other people, and default users do not deploy. Making the three doors and their boundaries
explicit keeps product copy honest about current capability versus roadmap, and keeps SaaS
from opening before its compliance and tenancy prerequisites exist.

Validation

Docs-only change; no runtime behavior change. Markdown reviewed locally; CHANGELOG and
README links checked against upstream main (rebased onto c3e8944).