docs, hive-c0re: retire prose describing the removed get_loose_ends target

Follow-up to 729c5b4f42, which dropped the
`agent` parameter from get_loose_ends and deleted Capability::QueryAgentState
with it. Three prose sites still describe the interface that commit removed:

- socket_server/mod.rs's module doc claimed authority on this socket derives
  from "capabilities for the hive-wide queries and tool-group membership for
  the orchestration verbs". There are no capability-gated queries left —
  `git grep -n has_cap -- hive-c0re/src/socket_server/` returns nothing, and
  require_group("scheduling") is the only gate in dispatch_orchestration.
- agent-hierarchy.md listed loose-ends visibility ("manager sees hive-wide,
  sub-agents only their own") as one of the manager-only overrides that
  "exist across hive-c0re today". It doesn't, and the sentence pointed a
  reader at loose_ends.rs for owner-check logic that is no longer there.
- conventions.md's Loose-ends wire shape still spoke of "the agent-flavour
  and manager-flavour requests" and "the agent-flavour list". There is one
  request shape.

No behaviour change: prose only.

Refs #4480

hive-c0re/src/socket_server/mod.rs

@@ -4,8 +4,9 @@
 //! you are `foo`; the manager socket simply serves as `ruth`. There is no
 //! privilege flag — both transports run the same [`serve`] / [`dispatch`]
 //! code, and authority derives uniformly from the caller's identity:
-//! capabilities for the hive-wide queries and tool-group membership for the
-//! orchestration verbs. An agent-targeting verb may name any agent, so `ruth`
+//! tool-group membership for the orchestration verbs, and nothing else —
+//! the queries are all self-scoped, so no capability is consulted here at
+//! all. An agent-targeting verb may name any agent, so `ruth`
 //! reaches every agent exactly the way every other agent does, not via any
 //! hardcoded name match.