docs: restructure the README's how-it-works into Roles, Artefacts, Process

[REPPL]

Restructure the how-it-works section: Roles → Artefacts → Process

Replaces the # How abcd works wrapper (Roles + Process) with four parallel
top-level sections — Roles, Artefacts, Process, Resources (the last already
existed) — and folds in the brief/intents mental model settled during the
brief-reconciliation work:

• Artefacts (new section): the brief's hard promise in lay terms —
  everything it says reads true right now; day-one ambition is visibly
  marked and each marking comes off in the change that ships the capability.
  The three-record division closes the item: the intent is the why, the
  brief the what is, the spec the how.
• Process: shipping now closes the loop twice (fidelity verdict onto
  the intent, marking off the brief); the refine/grill step is explicit in
  the flow diagram, sitting after acceptance criteria with the
  not-yet-clear loop returning into it; the capture hatch gets its own
  explanation (one line, un-typed, facilitator triages later).
• Retained from the old text: the reviewer's fail-honestly paragraph
  (delivered vs couldn't-tell is a distinction the framework insists on).

Prose-only; docs lint clean (0 blockers). No behaviour claims added beyond
what the shipped binary does (abcd intent "<text>", abcd capture "<text>",
ledger path, defaults — all verified this week during the currency work).