Admin-gated Settings: runtime model selection, is_admin enforced (#90)

Closes #79. Agent model + on/off move into an admin-only Settings section, and
is_admin is enforced for the first time. The live Runner is hot-swapped behind an
atomic.Pointer with routes always registered, so a settings change takes effect
with no restart; /capabilities reads the pointer. Secrets stay in the env, never
the DB. Precedence: Settings → env → default. Verified live: swap on/off/model,
bad spec → 400, race-clean. Gadfly blocking round addressed (dead code removed,
error-swallowing fixed, frontend 503 handling, holder dedup).

internal/api/agent.go

@@ -56,6 +56,16 @@ type stepEvent struct {
 }
 
 func (h *handlers) agentChat(c *gin.Context) {
+	// The route is always registered, so the assistant being off is a runtime
+	// state, not a missing route: answer it plainly rather than 404ing a path
+	// that exists. Loaded once here so a settings-driven swap mid-request can't
+	// make it flip between the guard and the Run call.
+	runner := h.agent.get()
+	if runner == nil {
+		writeAPIError(c, http.StatusServiceUnavailable, "AGENT_DISABLED", "the garden assistant isn't enabled on this instance")
+		return
+	}
+
 	var req chatRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
 		writeAPIError(c, http.StatusBadRequest, "INVALID_INPUT", "a gardenId and a message are required")
@@ -78,7 +88,7 @@ func (h *handlers) agentChat(c *gin.Context) {
 	stopBeat := stream.keepAlive(keepAliveInterval)
 	defer stopBeat()
 
-	turn, err := h.agent.Run(c.Request.Context(), actor.ID, req.GardenID, req.Message,
+	turn, err := runner.Run(c.Request.Context(), actor.ID, req.GardenID, req.Message,
 		replayHistory(history),
 		func(s mdagent.Step) {
 			send(chatEvent{Step: &stepEvent{Index: s.Index, Tools: toolNames(s)}})