docs: retire the agent hierarchy from every page that described it

The topology doc keeps its filename and its second half (manager
special-casing, harness unit shape) — both are cross-referenced from
other pages and neither is about the parent field. Its first half is
rewritten: what topology.json is now, and a table of what the removal
took with it, so a reader who finds `<parent>` or `set-parent` in an old
issue thread learns it went away rather than moved.

The dashboard's tree-rendering section is marked dormant rather than
deleted: the walk is still in swarm.js and retiring it is the frontend
owner's call.

docs/process/conventions.md

@@ -87,24 +87,13 @@ angle-bracket and asterisk shapes below are structurally safe.
   (`socket_server::handle_send` fans out via `Coordinator::broadcast_send`).
 - `operator` — the human at the dashboard. Messages accumulate in the
   inbox view; no agent ever `recv`'s them.
-- `<parent>` — the sender's parent per `topology.json`. Rewritten at
-  send time by `topology::resolve_recipient`: looks up
-  `parent_of(sender)` and falls back to `operator` when the sender is
-  a root agent (or absent from topology entirely). Lets agents address
-  their parent without learning the label, so runtime reparenting
-  propagates with zero agent-side restart.
-- `<children>` — fan-out to every direct descendant of the sender per
-  `topology.json`. Resolved in `socket_server::handle_send` via
-  `topology::children_of(sender)`: it delivers one message to each
-  child, bypassing the allow-list check (structural fan-out targets are
-  never user-listed peers). No-op for leaf agents (returns `Ok` when the
-  child set is empty). Lets a sub-manager nudge its subtree without
-  enumerating labels.
 
-When a `<children>` or `<parent>` send resolves to real recipients, the
-broker stores the *resolved* labels as the message recipients — the
-dashboard and recv side see the real routes. The sentinels are purely
-send-time addressing conveniences.
+`<parent>` and `<children>` were two more, resolved against a
+`topology.json` parent field. #4472 removed that field and both
+sentinels with it: address `operator` where you would have said
+`<parent>`, and name the recipients (or broadcast to `*`) where you
+would have said `<children>`. Nothing rewrites a recipient at send time
+any more — what an agent passes is what the broker stores.
 
 ## Wire protocol