Add himalaya + meli mail clients and Shedding Snake arcade game (#88)

Mail clients (internal/mailclients): launch himalaya and meli as
alternatives to the built-in AgentMail reader, pointed at the member's
mailbox with the same master IMAP / loopback-SMTP creds. They run
host-side (secrets never enter a pod), from a throwaway per-session
config that is deleted on exit; operators can override the config
template and argv via env. Wired into the hub ("Mail · Himalaya/Meli",
locked when the binary is absent) and the mail@ route
(ssh -t mail@host himalaya|meli).

Shedding Snake (plugins/arcade): a molting twist on Snake inspired by
cha.rlie.co/shedding-snake. The snake barely grows — each apple sheds
its whole body as a permanent field of scales you must not bite. Walls
wrap, speed ramps up, scales age fresh-teal → dusty-gray, and molt
counts feed a global "shedsnake" leaderboard. Grace period lets you
slither off a fresh molt.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

plugins/arcade/arcade.go

@@ -134,6 +134,15 @@ func newMenu(user auth.User, ctx plugin.Context) *menu {
 				return m, m.child.Init()
 			},
 		},
+		entry{
+			section: "BUILT-IN",
+			label:   "Shedding Snake",
+			desc:    "snake that molts — every apple sheds your skin as obstacles; walls wrap",
+			run: func(m *menu) (tea.Model, tea.Cmd) {
+				m.child = newShedSnake(m.user, m.ctx, m.width, m.height)
+				return m, m.child.Init()
+			},
+		},
 		entry{
 			section: "BUILT-IN",
 			label:   "Hangman",
@@ -152,6 +161,15 @@ func newMenu(user auth.User, ctx plugin.Context) *menu {
 				return m, m.child.Init()
 			},
 		},
+		entry{
+			section: "BUILT-IN",
+			label:   "Leaderboard — Shedding Snake",
+			desc:    "global top molt counts",
+			run: func(m *menu) (tea.Model, tea.Cmd) {
+				m.child = newBoard(m.ctx, "shedsnake")
+				return m, m.child.Init()
+			},
+		},
 		entry{
 			section: "BUILT-IN",
 			label:   "Leaderboard — Hangman",