feat(tui): collapsible sections in startup banner (skills, system prompt, MCP)

The TUI SessionPanel banner now uses collapsible \u25b8/\u25be toggle
sections matching the existing Chevron convention used for runtime
agent details. Skills, system prompt, and MCP server lists are
collapsed by default; tools remain expanded as the most actionable
info.

- tui_gateway/server.py: _session_info() now passes agent._cached_system_prompt
  through to the TUI frontend
- ui-tui/src/types.ts: added system_prompt?: string to SessionInfo
- ui-tui/src/components/branding.tsx: rewrote SessionPanel with
  CollapseToggle helper + per-section useState toggles

Default states: tools=open, skills=collapsed, system=collapsed,
mcp=collapsed. Clicking any \u25b8/\u25be header toggles that section.

tui_gateway/server.py

@@ -1413,6 +1413,10 @@ def _session_info(agent) -> dict:
         info["mcp_servers"] = get_mcp_status()
     except Exception:
         info["mcp_servers"] = []
+    try:
+        info["system_prompt"] = getattr(agent, "_cached_system_prompt", "") or ""
+    except Exception:
+        pass
     try:
         from hermes_cli.banner import get_update_result
         from hermes_cli.config import recommended_update_command