docs: repoint eighteen pointers whose section no longer exists

`check-doc-refs.sh` resolves the PATH half of a `docs/x.md::Section`
pointer and stops there. The section half rots the same way, and more
quietly: the file still exists, so every path-shaped check stays green
while the pointer names nothing.

Eighteen sites, five distinct pointers, each retargeted at a heading
verified to exist rather than at the nearest plausible one:

  docs/web-ui.md::Container row
    -> docs/web-ui/dashboard.md::Container row
  docs/web-ui.md::Shared terminal pane
    -> docs/web-ui/shape.md::Shared terminal pane
      Both sections moved out when docs/web-ui.md became a two-heading
      index. The path still resolves, which is exactly why nothing
      caught them.

  approvals.md::Helper events to the manager
    -> approvals.md::Helper events to the submitting agent
      Renamed with the manager special-casing removal; the pointer kept
      the old vocabulary.

  approvals.md::Migration from the pre-tag
    -> approvals.md::Startup migrations (older hosts)
      Same content, including the HIVE_SKIP_META_MIGRATION kill switch
      the citing comment names.

  agent-hierarchy.md::Current state
    -> ::Where the tree lives   (topology.rs, container_view.rs)
    -> ::Reparenting            (topology.rs's set_parent, host-sock)
      Split by what each site actually asks for rather than repointed
      uniformly: two want the format and the source-of-truth rule, two
      want the reparenting validation.

Three known-dead pointers are deliberately left alone:

  * `docs/integrations/forge.md::Sources` sits on a line PR #3927
    rewrites; fixing it here would conflict for no gain.
  * `docs/web-ui/shape.md::One unified channel` names real text that is
    bold inline rather than a heading — which of those counts as
    resolvable is the open question on #3922.
  * `persistence.md::Harness state files` should point at a heading
    whose own text contains backticks, and the backticked-pointer form
    cannot nest them. That is a limit of the convention, not a typo.

Comments only; no behaviour change. Refs #3922.

hive-host-sock/src/lib.rs

@@ -234,7 +234,7 @@ pub enum HostRequest {
     /// Move an agent in the topology tree. `new_parent = None`
     /// promotes the agent to root, `Some(name)` sets a new parent.
     /// Validation rules + bind-mount caveat documented in
-    /// `docs/agent-lifecycle/agent-hierarchy.md::Current state`.
+    /// `docs/agent-lifecycle/agent-hierarchy.md::Reparenting`.
     SetParent {
         child: Ident,
         new_parent: Option<Ident>,