manager: needs_login / logged_in / needs_update events + update tool

crash_watch grows two more state-axes alongside running/stopped:

- logged-in (claude session dir populated for the agent)
- up-to-date (recorded flake rev matches current)

per-tick transitions emit HelperEvent::NeedsLogin / LoggedIn /
NeedsUpdate. seed-on-first-tick semantics retained — nothing fires
on harness boot for agents that were already in their state. only
needs_update fires the 'stale appeared' direction; the resolved
direction is already covered by Rebuilt.

new mcp__hyperhive__update(name) on the manager surface: idempotent
rebuild via auto_update::rebuild_agent. transient-aware (Rebuilding)
so the dashboard shows the spinner. login intentionally has NO tool
— it's interactive OAuth, only the operator can complete it.

prompts + approvals doc + turn-loop doc updated. todo grows a
'show per-agent applied config in dashboard' entry (separate
follow-up).

hive-ag3nt/src/mcp.rs

@@ -240,6 +240,12 @@ pub struct RestartArgs {
     pub name: String,
 }
 
+#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
+pub struct UpdateArgs {
+    /// Sub-agent name (without the `h-` container prefix).
+    pub name: String,
+}
+
 #[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
 pub struct AskOperatorArgs {
     /// The question to surface on the dashboard.
@@ -400,6 +406,23 @@ impl ManagerServer {
         .await
     }
 
+    #[tool(
+        description = "Rebuild a sub-agent: re-applies the current hyperhive flake + agent.nix \
+        and restarts the container. No approval required — idempotent. Use when you receive a \
+        `needs_update` system event for an agent."
+    )]
+    async fn update(&self, Parameters(args): Parameters<UpdateArgs>) -> String {
+        let log = format!("{args:?}");
+        let name = args.name.clone();
+        run_tool_envelope("update", log, async move {
+            let resp = self
+                .dispatch(hive_sh4re::ManagerRequest::Update { name: args.name })
+                .await;
+            format_ack(resp, "update", format!("updated {name}"))
+        })
+        .await
+    }
+
     #[tool(
         description = "Surface a question to the operator on the dashboard. Returns immediately \
         with a question id — do NOT wait inline. When the operator answers, a system message \
@@ -512,6 +535,7 @@ pub fn allowed_mcp_tools(flavor: Flavor) -> Vec<String> {
             "kill",
             "start",
             "restart",
+            "update",
             "request_apply_commit",
             "ask_operator",
         ],