[codex] add Ralph Codex adaptation docs

[shetautnetjer]

What changed

• add the Codex adaptation docs, task records, test notes, and gap tracking files
• expand the local AGENTS guidance so the repo can be used as a Codex workbench without losing Ralph semantics

Why

• capture the current local Ralph adaptation work in a publishable form
• make the repo self-describing for future Codex work loops

Validation

• docs-only change; no runtime checks were required