docs: the trust-boundary pages keep the operator guidance, the mechanism moves to implementation

boundary.md keeps the model, the design rule, why network isolation
matters and host admin socket access; the hive-priv socket activation and
the per-agent socket directory move to 4-implementation/trust-boundary.
security.md keeps the agent trust model and the path-name rule; the nix
builds mode with its threat model, and the hive-c0re privilege separation,
move to their own pages there. Headings and text move as they were, with
one level shaved off the heading depth. Links, code and nix pointers, and
the docs index follow the moves.

hive-priv/src/main.rs

@@ -109,7 +109,7 @@ fn check_socket_activation(
 fn socket_listener() -> Result<UnixListener> {
     // hive-priv is ALWAYS socket-activated by the `hive-priv.socket` unit
     // (fd 3 via LISTEN_FDS). There is intentionally no self-bind fallback,
-    // so dev and prod take the same path; see docs/2-handbook/trust-boundary/boundary.md.
+    // so dev and prod take the same path; see docs/4-implementation/trust-boundary/boundary-mechanism.md.
     check_socket_activation(
         std::env::var("LISTEN_FDS").ok().as_deref(),
         std::env::var("LISTEN_PID").ok().as_deref(),