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/README.md

@@ -30,7 +30,7 @@ declarations.
   state machine, `flake.lock` validation).
 - **What state survives destroy / purge / restart?** →
   [`agent-lifecycle/persistence.md`](agent-lifecycle/persistence.md).
-- **Who can do what to whom — agent hierarchy and privilege?** →
+- **Who can do what to whom — the agent roster and privilege?** →
   [`agent-lifecycle/agent-hierarchy.md`](agent-lifecycle/agent-hierarchy.md).
 - **How does claude get its prompt, and what tools does it have?** →
   [`turn-loop/`](turn-loop/README.md) — the loop, binary shape, turn