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-agent-mcp/src/mcp/args.rs

@@ -131,19 +131,6 @@ pub struct CancelLooseEndArgs {
     pub id: i64,
 }
 
-#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
-pub struct AgentGetLooseEndsArgs {
-    /// Whose loose ends to list. Omit (or `null`) for your own. You may
-    /// also pass the name of any agent in your subtree — a child, a child's
-    /// child, and so on down — without any extra capability.
-    /// Pass any other agent name to inspect their threads — requires the
-    /// `query_agent_state` capability; without it the request is rejected
-    /// with an error. The `"*"` hive-wide value is not available on the
-    /// agent socket.
-    #[serde(default)]
-    pub agent: Option<String>,
-}
-
 #[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
 pub struct UpdateMetaInputsArgs {
     /// Flake input names to update (e.g. `["bitburner-agent", "nixpkgs"]`).