feat: first-class plugin architecture + hide status bar cost by default (#1544)

The persistent status bar now shows context %, token counts, and
duration but NOT $ cost by default. Cost display is opt-in via:

  display:
    show_cost: true

in config.yaml, or: hermes config set display.show_cost true

The /usage command still shows full cost breakdown since the user
explicitly asked for it — this only affects the always-visible bar.

Status bar without cost:
  ⚕ claude-sonnet-4 │ 12K/200K │ 6% │ 15m

Status bar with show_cost: true:
  ⚕ claude-sonnet-4 │ 12K/200K │ 6% │ $0.06 │ 15m

hermes_cli/config.py

@@ -211,6 +211,7 @@ DEFAULT_CONFIG = {
         "resume_display": "full",
         "bell_on_complete": False,
         "show_reasoning": False,
+        "show_cost": False,       # Show $ cost in the status bar (off by default)
         "skin": "default",
     },