docs: persistence and config changes become one operator page

The operator parts of the persistence and config-changes pages merge into
agents/persistence.md. The sqlite tables, state dirs, mounts and migrations move
to 4-implementation/agent-state/persistence.md, and the deploy path, meta flake,
deploy markers, startup migrations, root container, system messages and
autoupdate move to 4-implementation/agent-state/config-changes.md. Links, code
comments and nix comments that named the old sections point at the new pages.

docs/README.md

@@ -86,11 +86,14 @@ The host a hive runs on and the operator's tools for it. Start at
 One agent: its roster entry, state, config and page. Start at
 [`2-handbook/agents/`](2-handbook/agents/README.md).
 
-- **How do config changes flow from manager to operator to container?** →
-  [`2-handbook/agents/config-changes.md`](2-handbook/agents/config-changes.md)
-  (config PRs, the meta flake, system messages).
+- **How do config changes flow from agent to operator to container?** →
+  [`2-handbook/agents/persistence.md`](2-handbook/agents/persistence.md#config-changes).
+  The meta flake and system messages:
+  [`4-implementation/agent-state/config-changes.md`](4-implementation/agent-state/config-changes.md).
 - **What state survives destroy / purge / restart?** →
-  [`2-handbook/agents/persistence.md`](2-handbook/agents/persistence.md).
+  [`2-handbook/agents/persistence.md`](2-handbook/agents/persistence.md); the
+  tables and directory layout:
+  [`4-implementation/agent-state/persistence.md`](4-implementation/agent-state/persistence.md).
 - **Who can do what to whom — the agent roster and privilege?** →
   [`2-handbook/agents/agent-roster.md`](2-handbook/agents/agent-roster.md).
 - **What does an agent's own page show?** →