feat: wire Infisical secret store into API server and MCP tools

- Wire secretsManager in NewHandler() — instantiate InfisicalBackend
  when OIKOS_INFISICAL_SITE_URL is set (previously always nil)
- Add get_secret, list_secrets, set_secret MCP tools with nil-backend
  graceful degradation
- Add oikos secret get|set|list CLI subcommands for Infisical
- Fix Set() bug: create-before-update so new keys are created;
  add Type: "shared" to Update so it finds the right secret;
  disable SDK cache so Get returns fresh data after Set
- Clean enrollment response: remove fake infisical_client_id/
  infisical_client_secret stubs, store age key in Infisical for real

internal/mcp/server.go

@@ -47,10 +47,19 @@ func objSchema(props ...prop) *jsonschema.Schema {
 	return s
 }
 
+// secretBackend is the interface MCP tools use to access the secrets store.
+// Defined here to avoid importing the full secrets package (which brings in
+// the Infisical SDK). Mirrors the subset of secrets.Backend used by tools.
+type secretBackend interface {
+	Get(ctx context.Context, key string) (string, error)
+	Set(ctx context.Context, key string, value string) error
+	List(ctx context.Context) ([]string, error)
+}
+
 // NewHandler creates an http.Handler that serves the Oikos MCP server.
 // agentID is the Nomos agent entity UUID; tool calls are logged to agent_activity.
-func NewHandler(pool *db.Pool, token string, agentID uuid.UUID) http.Handler {
-	s := newServer(pool, agentID)
+func NewHandler(pool *db.Pool, token string, agentID uuid.UUID, sec secretBackend) http.Handler {
+	s := newServer(pool, agentID, sec)
 	handler := mcp.NewStreamableHTTPHandler(func(r *http.Request) *mcp.Server {
 		if token != "" {
 			if r.Header.Get("Authorization") != "Bearer "+token {
@@ -65,11 +74,11 @@ func NewHandler(pool *db.Pool, token string, agentID uuid.UUID) http.Handler {
 // toolHandler is the function signature registered via AddTool.
 type toolHandler = mcp.ToolHandler
 
-func newServer(pool *db.Pool, agentID uuid.UUID) *mcp.Server {
+func newServer(pool *db.Pool, agentID uuid.UUID, sec secretBackend) *mcp.Server {
 	s := mcp.NewServer(&mcp.Implementation{Name: "oikos", Version: "dev"}, &mcp.ServerOptions{
 		Logger: slog.Default(),
 	})
-	for _, t := range allTools(pool, agentID) {
+	for _, t := range allTools(pool, agentID, sec) {
 		s.AddTool(t.tool, withActivityLogging(pool, agentID, t.tool.Name, t.handler))
 	}