Document every public GKnnn diagnostic

[anusbutt]

Context

GraphKeeper diagnostics are stable and searchable, but users currently have to read source and tests to learn which command emits each code and what recovery is safe.

Scope

Add a concise diagnostic reference derived from src/lib/errors.ts, CLI call sites, and validator output. Link it from the README recovery guidance.

Do not change diagnostic codes or exit behavior.

Acceptance criteria

• Every public code family and stable exit code has a plain-language meaning.
• Each entry names the emitting command or validator and a safe next step.
• The reference distinguishes validation, prerequisite, operational, and internal failures.
• A documentation test fails if the reference or README link is removed.