hive-agent-mcp, hive-c0re, hive-sh4re: drop agent param from get_loose_ends

get_loose_ends now always returns the caller's own loose ends, for every
caller including the manager (ruth) — there is no separate manager
surface, ruth is a normal agent with different default capabilities.

- AgentGetLooseEndsArgs removed; get_loose_ends takes no args.
- Wire Request::GetLooseEnds collapses from an Option<String> target to
  a unit variant.
- hive-c0re's handle_get_loose_ends drops the "*" hive-wide branch and
  the subtree/capability resolver (resolve_agent_state_target); both
  are gone since there is no longer a target to resolve.
- loose_ends::hive_wide and Capability::QueryAgentState removed as
  dead code — their only callers were the two functions above.
- is_descendant_of is untouched (still used by lifecycle_handlers.rs
  and schedules.rs independently of this change).
- Docs updated: docs/turn-loop/mcp.md, docs/web-ui/dashboard.md,
  docs/process/conventions.md (Loose-ends wire shape + capabilities
  table), plus the doc comments in hive-core-agent-sock, mcp_config.rs
  and capabilities.rs that described the old shape.

Refs #4480

hive-core-agent-sock/src/lib.rs

@@ -63,16 +63,11 @@ pub enum Request {
     /// Non-mutating — pulls from the broker without delivering. The
     /// per-agent web UI uses this to render its own inbox section.
     Recent { limit: u64 },
-    /// Loose-ends view. On the agent socket: `None` = self; direct
-    /// children are always accessible; non-children require the
-    /// `query_agent_state` capability — rejected with an error otherwise;
-    /// `"*"` is always rejected (use the manager socket). On the manager
-    /// socket: `None` = manager self, `"*"` = hive-wide, any name =
-    /// that agent. See `docs/process/conventions.md::Loose-ends wire shape`.
-    GetLooseEnds {
-        #[serde(default, skip_serializing_if = "Option::is_none")]
-        agent: Option<String>,
-    },
+    /// Loose-ends view — always the caller's own. Same shape on the agent
+    /// and manager sockets; the manager (`ruth`) is a normal agent here, it
+    /// just sees its own threads like anyone else. See
+    /// `docs/process/conventions.md::Loose-ends wire shape`.
+    GetLooseEnds,
     /// Set a free-text status string visible on the dashboard. The harness
     /// writes `{state_dir}/hyperhive-status` locally before sending this
     /// request; hive-c0re just triggers a dashboard rescan on receipt.