feat(#1082): add description() to ToolGroup + Capability; expose in API + UI tooltips

- hive-sh4re: ToolGroup::description() and Capability::description() return
  short human-readable strings for each variant
- hive-c0re: ToolGroupsSnapshot and CapabilitiesSnapshot now include a
  `descriptions` map (name → description); get_capabilities now iterates
  Capability::ALL instead of hardcoding the list
- tabs.js: renderToolGroups + renderCapabilities use descriptions[name] as
  the column header title attribute (native browser tooltip on hover)

hive-sh4re/src/lib.rs

@@ -876,6 +876,30 @@ impl ToolGroup {
             Self::WebTools => "web_tools",
         }
     }
+
+    /// Short human-readable description suitable for a tooltip or help text.
+    #[must_use]
+    pub fn description(self) -> &'static str {
+        match self {
+            Self::Messaging => "send, recv, ask, answer — core agent communication",
+            Self::Meta => "set_status, get_agent_meta — identity and status",
+            Self::Inbox => {
+                "get_loose_ends, cancel_loose_end, remind, request_next_turn — self-scheduling"
+            }
+            Self::Lifecycle => "kill, start, restart, update — container lifecycle (privileged)",
+            Self::Approvals => {
+                "request_init_config, request_apply_commit, request_update_meta_inputs — config change flow (privileged)"
+            }
+            Self::Scheduling => {
+                "request_schedule_prompt and related — operator-visible scheduled prompts (privileged)"
+            }
+            Self::Diagnostics => "get_logs — read host journal and build logs (privileged)",
+            Self::Execution => "bash_run, bash_status — run shell commands in the container",
+            Self::WebTools => {
+                "WebFetch, WebSearch — Claude built-in web egress; not MCP tools"
+            }
+        }
+    }
 }
 
 /// Per-agent capability grants. Stored in `meta/capabilities.json`
@@ -962,6 +986,20 @@ impl Capability {
             Self::QueryAgentState => "query_agent_state",
         }
     }
+
+    /// Short human-readable description suitable for a tooltip or help text.
+    #[must_use]
+    pub fn description(self) -> &'static str {
+        match self {
+            Self::ManageRootAgent => {
+                "lifecycle-manage the root/manager agent on hive crash recovery"
+            }
+            Self::ReadHostJournal => "read host journald via get_host_journal MCP tool",
+            Self::QueryAgentState => {
+                "query non-child agents' loose ends and reminder state via get_loose_ends"
+            }
+        }
+    }
 }
 
 /// Schedule row shape on the wire — mirror of