docs: spell out the turn and round model

doc/turns-and-rounds.md defines the two nested units the loop is built on: a round is one request→response plus the tool batch it asked for, a turn is one user message through the explicit stop (0..N rounds), and a session is a stored sequence of turns. It records what bounds each unit (max_rounds, truncation continues, retries), which work is round-scoped (request assembly, context reduction, the aggregated soft confirm) and which state resets per turn (read-before-edit, yyy auto-continue, /yolo once, the todo touched flag, usage totals) versus what outlives it (rows, todo stack, grants, reminder bands).

Linked from the architecture doc, with the short version in AGENTS.md.

doc/architecture.md

@@ -37,4 +37,6 @@
     - web: Web service (Phase H; not implemented).
 
 Plugins (third-party entry points, allowlisted under ``[plugins]``): [plugins.md](./plugins.md).
+Conversation units (a *round* = one request→response plus its tool batch, a
+*turn* = a user message to the explicit stop): [turns-and-rounds.md](./turns-and-rounds.md).