docs: add greenfield adaptation protocol

[dapi]

What changed

• add a compact Codex-executable protocol for adapting Memory Bank to a greenfield GitHub project;
• instruct Codex to copy memory-bank/, extract confirmed product and project facts from repository instructions, README files and docs, and place them in the correct canonical owners;
• require creation of an initial PRD from the discovered product description;
• add a copy-paste Codex CLI command to the adoption guide;
• link the protocol from the root documentation index.

Why

The repository explained the template and its individual sections but did not provide one direct task that an agent could execute to perform the initial adaptation. The new protocol stays focused on that job: copy, inspect, adapt, create the PRD, update navigation, and lint.

User impact

A user can open a greenfield repository and run the documented Codex command. Codex then adapts Memory Bank from the facts already present in README and docs without inventing missing requirements or starting product implementation.

Routing record

Workflow: Small Change

Design: not required
Reason: the change extends the existing adoption documentation pattern and introduces no runtime, API, configuration, schema, or release contract.

Plan: not required
Reason: the change is an atomic documentation update across one protocol-prompt and its navigation owners.

Validation profile: documentation
Triggers / rationale: Markdown-only public guidance; executable behavior and release path are unchanged.
Downgrade approval: none

Validation

• go run ./cmd/memory-bank lint
• go test -count=1 -race ./...
• go vet ./...
• git diff --check