docs(gateway): describe what is, not what changed

Per review: docs represent current state. Every "used to" / "no longer"
clause this branch introduced is gone — including the History section in
network.md, which was a whole subsection about a sync mechanism that
doesn't exist.

Where the removed clause was carrying a real constraint, the constraint
stays and is stated in the present tense instead of as a delta: nothing
narrows what the gateway's nginx can reach except the directory
permissions in front of a socket, and nothing bounds `ReloadGatewayNginx`
except the hard-coded unit name. Those read as rules now rather than as
the story of how they came to be rules.

hive-c0re/src/workers/agent_sockets.rs

@@ -14,11 +14,10 @@ use std::path::{Path, PathBuf};
 use anyhow::{Context, Result};
 
 /// Host-side parent directory holding per-agent socket subdirs. The
-/// gateway's nginx runs on the host and reads this whole tree, so it
-/// can `proxy_pass` to any agent — it used to get there through a
-/// read-only bind-mount of the same tree. Each agent's container
-/// bind-mounts only its own `<name>/` subdir, which is still what stops
-/// one agent reaching another's socket. The literal lives in `hive-host-sock` (shared with
+/// gateway's nginx runs on the host and reads this whole tree, so it can
+/// `proxy_pass` to any agent. Each agent's container bind-mounts only its
+/// own `<name>/` subdir — that mount is what stops one agent reaching
+/// another's socket. The literal lives in `hive-host-sock` (shared with
 /// `hivectl`); re-exported here under the name this module's consumers
 /// have always used.
 pub use hive_host_sock::AGENT_SOCKET_DIR;