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.

hive-c0re/src/stores/build_logs.rs

@@ -1,7 +1,7 @@
 //! Sqlite-backed full build-log capture — stdout + stderr per
 //! `nixos-container` / `nix build` invocation, accumulated live.
 //! Schema, indices, retention, and the rationale for replacing
-//! the old ring buffer: `docs/2-handbook/agents/persistence.md::/var/lib/hyperhive/db/build_logs.sqlite`.
+//! the old ring buffer: `docs/4-implementation/agent-state/persistence.md::/var/lib/hyperhive/db/build_logs.sqlite`.
 
 use std::path::Path;
 use std::sync::{Arc, Mutex, OnceLock};