Add per-agent run stats (messages, tool calls by type)

compute_run_stats() walks the conversation AST after each agent
completes, counting messages and tool calls by tool name. Stats
are returned from save_agent_log(), stored on UnconsciousAgent,
and displayed in the agent list UI.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/subconscious.rs

@@ -250,6 +250,10 @@ impl SubconsciousScreen {
                     format!("run {}", snap.runs + 1)
                 } else if !snap.enabled {
                     "off".to_string()
+                } else if let Some(ref stats) = snap.last_stats {
+                    format!("×{} {} {}msg {}tc",
+                        snap.runs, ago,
+                        stats.messages, stats.tool_calls)
                 } else {
                     format!("×{} {}", snap.runs, ago)
                 };