docs: describe the context checkpoint belief model

AGENTS.md

@@ -159,4 +159,5 @@ const table = sqliteTable("session", {
 - Keep local Session drains process-local until clustering is implemented. `SessionRunCoordinator` joins explicit same-Session resumes, coalesces prompt wakeups, and allows different Sessions to run concurrently. Advisory wakes drain eligible durable inbox rows only; post-crash continuation recovery requires a separate explicit design before it may retry provider work. A drain has no durable identity or transcript boundary.
 - Keep delivery vocabulary explicit. Prompts steer by default and promote at the next safe provider-turn boundary while the current drain requires continuation. An explicit `queue` input remains pending until the Session would otherwise become idle; promote one queued input at that boundary, then reevaluate continuation before promoting another. Promoting any new user input resets the selected agent's provider-turn allowance; a batch of steers resets it once.
 - Keep EventV2 replay owner claims separate from clustered Session execution ownership.
-- Keep the System Context algebra, registry, and built-ins in `src/system-context`; keep Context Source producers with their observed domains, and keep Session History selection plus Context Epoch persistence Session-owned.
+- Keep the System Context algebra and built-ins in `src/system-context`; keep Context Source producers with their observed domains, and keep Session History selection plus Context Checkpoint persistence Session-owned. The runner composes all context producers explicitly in `loadSystemContext`; there is no context registry.
+- The durable Applied record is what the model was last told, per source. Reconcile narrates drift as chronological System updates and never rewrites the baseline; only completed compaction rebaselines, and move or committed revert resets the checkpoint. Unavailable sources keep the model's prior belief, blocking only a session's first baseline.